I want him to stop but I don't know how to tell him.

I have a friend whom I have been very close to. He is not my best friend but the two of us just mostly spend time together ever since we were in college. We have been very close for so long that I got used to everything he does. However, he was acting strange recently toward me. I got used to him being nice and all but he also annoys me quite often than not. This time, he is all but nice and when I got to ask him about it, he confessed his feelings. Honestly, he is a great guy, I swear, even if he used to mess around during college. It