Where should we do the x-rays in this situation?

If it were me I'd want spinal x-rays.  some dogs are stoic and you won't get them to flinch when prodded/palpated.  You may find some vertebrae have fused from an old injury -- he may not bend because he can't.  I'd probably tell the vet to do extras if they find some *thing* that begins to look likely and they need to do an extra while he's out.

rwbeagles

not really wanting to sit "formally" is typically a sign of tight pretty hips actually. Most Greyhounds for example cannot do that, because their hips are THAT tight. The breed has one of the least incidences of HD in any breed. I would be thinking, knees, because of the breed...and possibly because of the pooping issue...issues with the sphincter, pain perhaps...or even pelvis. Yes the spine could be an issue...but it would seem to me that if there is no real other incidence of pain or discomfort...like leaning down to eat...climbing stairs...scratching an itch...nibbling at their hind end like all dogs do sometimes...seems unlikely. What's the poops like? Constipated? any rectal bleeding? Straining? growths or bulging in the rectal area?

Interesting on the hips, and the sit so thanks!  Yes bichons are prone to luxating patella, his knees checked out fine in the exam (and I'd had them check by a different vet when I got him 2 years back).  The hip dysplasia was the other thought; my RB Marlin had very bad hips.  But again seems OK at exam.

We have a ranch house, no stairs to speak of, so I don't know how he is on them.   To go out back, we go down one step, walk through the garage, then down one step to the grass, done.  For walkies we go down the one step into the garage and out the big door.  I do it this way for the safety of the 'double door" system, as I also have cats and 3 are door darters.  So we don't use the front or side doors much at all and never with the dogs.  So stairs are not something I can easily observe with him.  He has never shown any interest in going down cellar with me when I do laundry, but I don't want any of them down there and only Tasha has ever been down.  So no help from that.

 Poops are normal, firm not hard, no bleeding or straining, no bulging.  

I, too, would add x-rays of the spine and possibly shoulders.  

 I spoke with the vet on the phone this morning (called to say lab work is OK, he can have the anesthesia) and added the spine.  We discussed shoulder, funny enough, but she couldn't see how that would connect?  Could you explain your thinking for me?  Thanks.

Just from my own experiences (human, not dog), I know that pain in one area of my back/neck, etc., is usually an indicator of a problem that started else where either up or down the spine, often quite far away from the actual pain site.

 OK, thanks.  When I spoke with the vet on the phone this AM, that was prior to reading your post.  But due to our discussion she did say she would check his shoulder.

Funny thing, last February I had the opportunity for a free consult with a pet communicator.  At the time, Willy told her his hips and knees are fine but his left shoulder gets very sore in Agility class.  This is why I mentioned shoulder in the phone call!  I don't normally put much stock in those folks, too hard to tell if you have a good person or not.  Still, many of the things she said Willy told her were right on target, it was fun and interesting, at least.  He goes in next Friday so I can think / research on the possible shoulder connection to this.  Thank you again.