Hi, I'd like to help you with your question.
It sounds like there could be two things going on here. One, there is something bothering him personally and he doesn't want to share it, or two, there is a problem in the relationship and he is not letting you know what it is.
It is very hard to tell what could be bothering him unless he is willing to tell you. You have tried all the important steps like asking him directly and setting boundaries. But if he still is not responding, then there is little you can do but respond on your own.
Since he will not tell you what is going on, you can do two things. You can keep asking him to see if he will tell you, or you can ignore it. If you choose to keep asking him, then sit him down and tell him that his behavior is starting to interfere with your relationship. Tell him communication is the most important part of keeping your relationship healthy and that you would like to keep it that way. And then see what he says. He may choose to talk about it or not.
If you choose to ignore his behavior, then he may end up telling you. Sometimes people like the control of withholding information from others and when the attention they get from it goes away, they stop withholding. Be careful here not to move away yourself emotionally, however. Pay back rarely works and usually ends up damaging the relationship.
Keep in mind it may be just his nature to not share everything in his life with others. He is telling you that he cares for you and loves you. If he shows no other signs of straying or hurting you otherwise, there is no reason to worry. Give it time and see what happens.
I hope this has helped you,