I think its very kind of you to want to help this chap.. and I think I would want to do the same.
Is there a way that you can help, but just a bit? If your husband liked him and would be happy to do so, perhaps meet for a drink once in a while, join him on a walk or a light lunch & chat in a cafe? Its a thin line of offering the hand of friendship and him relying on you if he is lonely, but I think it may be worth the risk. He knows you're married and he's met your hubby, so surely that shouldn't be a problem? Him being initially disappointed that you're hitched doesn't have to equal someone who can't move on and just be friends. As long as you don't send any signals out, I don't think there should be a problem. What does your gut tell you?