Stop trying to force feed him for now. It's adding a lot of stress for no reason at this point. A couple of months without food is not a big deal for a BP.
How much weight has he actually lost? If you aren't weighing him, you need to get a scale to track his weight. Guessing at weigh loss on a snake can be difficult. They go 4 weeks without food, we think they're losing weight while, in fact, they will have lost something trivial like 4g.
You said ambient temps are 90 on the hot side. What are the temps on the ground where your snake is in contact with the substrate?