Hope he's ok by now. I don't know where you live but where we live in central Tx, my husband got stung first by a wasp and then 5 min later on the other hand by some kind of spider, not a widow or recluse, but wolf or writing spider. He used asuction device to extract the poison but even so the next day his hand was quite swollen. So it could have been a spider. It could swell his trachea (windpipe) and he could experience trouble breathing.
He could have had a bee or wasp sting in his mouth.
But I'd say if it is affecting him very badly and he is looking worse by the minute, to take him to be seen by the vet.
could be some sort of insect bite if it got swollen call your vet try some benadryl see if that helps dogs do not eat or play when in pain so call the vet it could also be have more then a local reaction to what every it is.