If money is a concern you may want to way out what it might cost you to continually use an antibiotic and that vets fees - versus possibly seeing another vet that could give you a solution that might up front cost more, but take care of the skin issues.
Also, what does the use of the antibiotic do to your drug over the longterm - does it shorten his life? Those are also important things to consider.
Money is a concern for me too, and it is so hard when we have no idea what is going on with our pets. And we feel like it is hard to know if the vets really know
-- but right now, you know the treatment he has had is not working for long - so something different needs to be done. Longer course of medicine or a different med might be it