At this point, force feeding would probably make the situation worse. It would be adding stress to an already stressful situation, with the RI.
Keep the humidity up!!! Too-dry conditions can cause RIs just as easily as too-wet conditions. Switch out the aspen for paper towels or newspaper so that you can more easily regulate humidity. Shoot for 60 or 70%; the substrate should never be wet, but it is important that your snake's lung does not dry out. Rinse and refill his water dish daily.
Beyond that, the best advice I can give you is to hang in there and don't lose hope. It sounds like he is not in good shape, but six months without food is not going to kill him. Ball pythons are resilient little guys. Bring your concerns to your vet, keep up treatment, and this mess will resolve with time!