I am glad Gavin is feeling better, however the bright red on his skin is what is the concern. I would have guessed it was some type of allergic reaction too.
I have a young Yorkie who gets into everything all the time too, and it is nearly impossible to keep things from dropping on the floor in a busy household. things happen accidently . I would keep communicating with your vet and bring Gavin in again . Something is causing the redness that needs to be addressed as soon as possible. Best wishes to you and Gavin.