So, ya. I'd think about a counselor. A social worker, even. Obviously a teenager humping on a parent is problematic. That is, if you are sure what he is doing. Does he have any developmental issues? High functioning autism for example? It would be more common with that. I think a social worker who works with families (you and your son both) to discuss how to handle this and boundaries. We have been to a family social worker and they do a really great job. Is that something you could consider? I think through a local children's hospital can be a good place to start. They normally have counseling centers and can work with you if you don't have insurance. Let me know what you think. This would be what I do.
Seriously, if your son is humping anything but a willing female near his age, as in dating, he needs help. Your situation doesn't sound great for therapy, but something is going on here and it needs to be dealt with quickly, I would think.