Husband wants to work out

My husband had recieved injuries to his back, hip, and spine while in the military army. He's expresses that he wants to work out again and I offered to help him as much as possible. But due to these injuries I know he's at a stand still at the moment. I wanted to ask you guys what work outs could he do that won't hurt him. He's stubborn and would never tell me if something hurt. He already does physical therapy and goes to the doctor. He is just trying to grasp something again and I wanna help 1000 percent.

added info: I know this may sound silly, but thank every single one of you. I have been married to this man for almost a decade and known him since we were kids. So I really want to help my best friend. I really appreciate all of you coming with advice and information. I feel overwhelmed with gratitude. Thank you so much!