Help from anyone with Bearded Dragons!

How old is the dragon?  I don't have one myself, but a ball python.  I've gone the route of antibiotics for him twice in his lifetime but he wasn't near death or anything.  I would be tempted to talk about all this with the vet.  Find out costs, what's needed, etc, then decide.

I hope someone with experience can answer your questions...

The dragon is about 3 years old...about 18 inches long.

Leslie, I've had a few, and currently have some dragons, so I hope I can help you out. I would not have him put down without trying the following first:
Give him a warm bath. I'd go with about what temperature you would use for a baby, maybe slightly warmer. Put enough water in to get him swimming, as it tends to get things moving. If you're lucky and the impaction isn't too bad, this will make him poop.
If not, other things to try are rubbing his belly. Rub in the direction that you want the stuff to move in. Don't put in so much water that he's floating in it, but just enough to make him swim. He might flail around like a maniac, but don't worry about that. In your case this is actually a good thing.
Also, try making a little donut ring out of a towel, and let his belly hang in the center and in a warm bath. In this case, you don't want to give him enough water to swim.
Getting him to drink a lot would also be good. If he won't drink a ton of water, you can try using a dropper to drip juice on his nose.
I would also try increasing the temperature of your basking spot (which is the temperature of the hot spot in your cage). Get a good thermometer if you don't already have one, because you don't want to cook him. What is the temp there right now, and if it's not already at the high end of the range for beardies (95-105) can you get it up to there? If it is already at 105 or so, you won't hurt him to go up to 110.
If it's warm where you are, get him outside. Do not use a glass tank. If you can get a reptarium (it's a mesh cage) that would be great. If you cannot, get a leash, which is available in most petstores, but be very careful that he does not eat anything outside. If he eats a firefly, that is absolutely, 100% fatal, and I can give no advice for it. If you lived close, and i'd lend you a reptarium since I have quite a few, but I see you're on the opposite side of the country.

Also,  what substrate are you using? If you are using sand, get that out right away, and switch to paper towels.
And... what size crickets are you feeding? Are they small than the space between his eyes on the top of his head? Are you feeding anything else?
Finally... you might want to try the forums at www.kingsnake.com . Last time I posted there, they were very good, but I must say that I haven't frequented them for years. Nonetheless, you might find some more people and more advice to try there. I hope your little dragon is able to make it. Let me know if you have any questions about what I suggested, and i'll try to clarify for you.

Leslie, I know this probably is not what you want to hear right now, but if you are using that horrible product calci-sand (the expensive sand in the petstore that claims to provide your pet with calcium) I would suspect that as the cause of the impaction, and I would not be too hopeful if that is the case. *begin rant* It's a horrible product, doesn't meet the calcium requirements for your pets, and doesn't pass out of their stomachs if ingested. It will collect in there forever. Dragons should not be eating their substrate to begin with, and those who do should not be kept on sand, or any other type of particulate substrate. They shouldn't be allowed to sell that crap. *end rant*

I've had beardies before and griffinej5 gave some great advice. Especially the warm bath part. My dragons would poo in their bath EVERYTIME they had one....even if they went earlier that day!!

Thanks Jen and all for your advice...
Well it turns out my internet diagnosis was wrong.  The vet x-rayed Spike, and he has a calcium deficiency.  Both of his hips have collapsed.  The vet said that the bones in his feet don't even show up in the x-rays because they are so tiny and weak.  He did say that Spike can recover.  He gave me some meds (some sort of calcium supplement, I think) to give to him for thirty days, told me to remove anything he can climb on his cage, to force feed him if he doesn't eat, and to get better lighting.  So...$191 spent at vet and $150 spent at Petco.  If this lizard ends up dying, I'll be most upset.
Any other helpful tips to get this beardie on the road to recovery?

I'm glad he's got a chance of surviving it. In addition to better lighting, which is a must, natural sunlight would be great for him. I've got two of mine out on the deck right now. It's gonna be even more money, but I would highly recommend one of these [linkreviousText=reptarium&N=2]http://www.petsmart.com/global/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524441777365&FOLDER%3C%3Efolder_id=2534374302030170&ASSORTMENT%3C%3East_id=2534374302023694&bmUID=1146862592566&itemNo=0&Ntt=reptarium&In=All&reviousText=reptarium&N=2[/link]>http://www.petsmart.com/global/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524441777365&FOLDER%3C%3Efolder_id=2534374302030170&ASSORTMENT%3C%3East_id=2534374302023694&bmUID=1146862592566&itemNo=0&Ntt=reptarium&In=All&reviousText=reptarium&N=2]http://www.petsmart.com/global/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524441777365&FOLDER%3C%3Efolder_id=2534374302030170&ASSORTMENT%3C%3East_id=2534374302023694&bmUID=1146862592566&itemNo=0&Ntt=reptarium&In=All&reviousText=reptarium&N=2[/link]
so that you can leave him outside and not risk him eating bugs. If you could get him out for even an hour or a day, that would be great for him.  If he isn't getting adequate UV, the calcium is going to be useless. The UV from the sun, even on a cloudy day, is much more than you could ever provide with a light. The 22 would be on the small side for him, but not too big a deal, since he would not be living in it. I've got 2 dragons outside right now, and I need to buy some more reptariums so I can put them all out. I thought I had 4, but I only have 2 in usable condition at the moment.
As for if he won't eat, force feeding is great fun. If you have to resort to that, babyfood meat would be the easiest thing to use probably. Be careful about it if you actually have to do it. Shooting food down his throat could cause him to aspirate it into his lungs (sorry, I may not have my terms correct here). Just put a little in the side of his mouth, he should lick it up. You could also thin it a bit with water (although juice or something like that would be better) and try dripping it on his nose. I have to go, but I will have more later.