Absolutely everything Janet said ... but give the Benedryl FOUR times a day (you'll probably have to wake him up to give it to him) each day for at least today and probably tomorrow.
This is called "anaphylactic shock" -- it can *be* life-threatening IF the airways swell shut -- so FROM NOW ON you keep not just Benedry, but the children's "elixer" (liquid benedryl) on hand. The elixer acts FASTER and in the case of a dog who swelled this fast you may need it.
The OTC caps are 25 mg. usually
The elixer is 12.5 mg a teaspoonful. That means you give a big dog a TON of it (so it seems). Literally you'd give a 75 pound dog SIX TEASPOONFULLS at once.
but it hits the bloodstream faster and can literally save their life. I have, upon occasion with a big dog who was swelling before my eyes, literally just opened their mouth and POURED. Why? Because the 2 mg dose is actually still quite low -- a vet can and *will* exceed that to respond to an emergency. And frankly, the dog's throat, proportionately, won't really allow them to swallow more than is safe, IN THIS CASE.
My point is this -- the **second** time can be worse than the first. Just like humans dogs can develop extreme allergies to things like bee stings, wasp stings, certain contact allergens (like plants) and even the bites of spiders.
Snake bite will swell but not like that. and you'll SEE fang marks.
One night a dog I had let out literally 2 minutes before came in COVERED with "hives".
Hives in a dog? Yep -- it looks like someone hit their fur with spatters of water and they 'shook' and SOME of the hair is standng straight up and some is lying down. It looks literally like someone flicked water on them.
Socks was covered in this raised skin from the tip of her nose to the back of her neck and down her chest to her paws. THAT fast. *I thot it was red ant bites* -- but it was so late I couldn't figure it out. but that was the time I upended the bottle of benedryl in her throad -- she must have had 5-6 spoonfuls (by the time she hacked some back OUT at me). No, dearheart I didn't drown you!!!
Seriously -- ALWAYS keep Benedryl now, and make sure every human who takes care of this dog always knows it's reactive to SOME sort of insect.
We never did find out what Socks got into that night.
Good luck -- this is the scariest stuff because it happens SO fast. Just be aware it can happen AGAIN, fast (particularly if there is a nest or something out there).