Help for pet care for low income families?

We’re in kind of a predicament where my BIL abandoned his dog (7 year old pitbull mix) by leaving him when he moved out. He also neglected him where I don’t think he’s taken him to the vet since he was a puppy and got his initial vaccines and got neutered. I really want to take him to the vet to get a comprehensive exam, but my husband and I aren’t really in the financial place to afford it. I’m still looking for a job and depending on my husband’s income ($18/hr), and we already have two cats. One of them has a pre-existing condition and while all we need to do is feed him prescription food, it’s still pretty expensive.

Is there any low income programs you know where I can at least get him affordable vaccine boosters, especially rabies? I just want to make sure this dog is properly taken care of.