Has he been outside alot? could be a reaction to a fly bite or even a tick bite ( lyme disease possibly).
However, i'd place my money on a fly bite.

 

He is outside for walks and out in the backyard some during the day. Thanks for your reply I've been quite worried, will see a vet fri.

 

Sounds like it could be lyme disease from a deer tick bite. Antibiotics from a vet will fix it right up. Wisconsin is an extremely bad area for infected deer ticks so make sure you check your buddy regularly and use some flea/tick preventatives.