What a load of crap. It's totally possible to be thin and stable no matter what meds you're on. Will it be easy to lose weight? Depends on the meds. Might be very easy or extremely difficult, but it's still always possible no matter what. So for your doctor to say you may reach a point where it's not possible, that's a load of crap IMO.

How could someone in your situation lose weight? Depends on the person and the meds. You could try a nutritionist to keep you on track, or you could find ways to change your lifestyle to help keep the weight off. I don't know how you'd change your lifestyle, but it may mean you have to exercise twice as much as the average person, which totally sucks, but it could work for you.