Exercise for someone who is crippled?

My fiance has a bad back, bad knees, and can't do strenuous exercise without being in extreme pain. He's not fat or anything, I'd say average at the most. But lately he's been down emotionally and I've read that exercise is good for depression. For the past three weeks he's been working almost every day. At first I thought that that was awesome-- he likes his job and he's making more money. But he's in pain during the whole time he's there (lately he's been painting walls). So I suggested taking a day off and he says that he would feel worse just sitting around all day. He says he needs purpose. I thought that exercising would help him. He's always talking about how he would love to go diving and take swimming cles. That's good, right? He's tried signing up at the YMCA a couple of times but they're not being very helpful. Any advice?