This sounds like a serious allergic reaction to me. He may have been stung or bitten by an insect causing this.
You can try Benadryl at a dose of 1mg per pound of dog so a 50 pound dog would get two 25 mg tablets for example.
Hopefully you are headed to the vet with him and they can give him better allergy meds. If you do use the Benadryl tell the vet when you get there that you did so.
Hope he is OK!
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