Sorry, but if you can afford Vet care for your dog, maybe you took on too much responsibility and it is a responsibility.
We are not Vets and can only advise. It could be a number of things, one being a bad tooth infection. Have you checked his mouth?
A Vet really needs to examine this as he may need Vet care and antibiotics
Check into Care Credit. Call your Vet, most accept it.
This really does need to be checked!.