You have nothing to lose by sharing this openly with him. Some sensitive men get it in their heads that they are doing something wrong if they want to be intimate with someone who is frail for whatever reason. My spouse is like that. He is overly concerned that everything is okay and he's not forcing etc.

The other suggestion I have is to look for M.S. support groups. It's an illness with many ups and downs. can't hurt to share with others who know. Good luck.