Suggest Treatment For Lack Of Movement In Thumb After Ulnar Fracture

I had surgery to repair a nonunion fracture to my ulna. It has been 5 weeks since the surgery and now I cannot lift my thumb. I can move it some (to make a fist) I just can't make it straight. The rest of my fingers move fine, except for my thumb. I have osteogenesis imperfecta and I am not able to walk. My hands/arms are like my legs too. What would you suggest? The doctor is fully aware of it and he says he can just move a tendon over. Is it really as simple as he is making it sound?