Please take him to be checked by a vet. Hopefully these could just be bug bites, stings. But it may be an allergic reaction to something. And it might be a flea allergy. If they are flea bites, those can be quite devastating for a dog and can, if there are enough of them, sometimes lead to anemia.
If he sleeps outside, he may be more prone to getting bitten by insects.
Or it might be as a result of a food allergy perhaps?
It might NOT be those causes. He could have them for some other reason, or even some serious reason, such as mast cell tumours.
So the best and most sensible thing to do, is to get him checked over by a vet.
Hi there. Wow, poor pup. Have you taken him to the vet as of yet? They can get into things and maybe your dog did or maybe it is cell tumors. Sure hope not. Give us an update.