I'd bring him to the vets just to be on the safe side. And call the breeder if you are worried, see if any other puppies have come down with anything. If he looks that skinny and being a puppy I'd personally reccomend that you bring him to the vets.

Is he drinking a lot or at least his normal amount of water?

When did he get his vaccines?
An allergic reaction usually happens within a few minutes to 24 hours tops. And also if he had an allergic reaction, 99% chance somehting would swell (skin, eyes, lips) and may also get hives, fever, lethargy. It could be some other reacton though and not an allergic reaction or a weird allergic reaction.
If it was a reaction to the vaccines & he's not better by 24 hours it is reccomended that they be brought back to the vets, even if it is a mild reaction.

Hopefully he just got into something that wasn't that harmful & will be back to his old self in no time.
I wish I could be of more help. Good luck sweet Tyson! Keep us updated please!