Well, Monica, it does sound as if he is either cheating, or at the least, no longer interested. You could confront him, but he will deny it. I would still tell him that you suspect he is seeing someone else, and If you must, that you will hire a private investigator. Tell him, you want to go to counseling, NOW. Do it and confront him there. At the least you may be able to get at the reason for the changes. Best wishes, I sure hope your suspicions are wrong!
Dan