Just because of where I live, I always, always add tick diseases/infectious causes to my lists of things that sound painful.
Tick diseases can have such vague and nonspecific signs, that I always worry about that when a dog is seeminly painful and down and out.
Although aspirin is going to help make him more comfortable it is a muscle/bone issue, if it is tick related, or pancreatitis, or something else, it may only be a temporary fix.
If he seems better, it certainly warrants watching him closely (as I know you are
) and seeing how he does, but at the slightest backstep, I'd have him checked out. Not that I need to tell you that, but for completeness' sake!
I sure hope the big guy is feeling better!