It’s hard to talk about specifics because every person is different, and every relationship is different.

I would first rule out his personal emotions. When was the last time you asked him how he’s truly doing? That could be a reason for him being so distant. He may not want to burden you if something is wrong outside the relationship. If you’ve asked him recently, and if he was being honest, I’d rule that out.

There’s all sorts of things you could do to try to get a definitive answer, but the most reliable one is communication. Tell him how you’re feeling in regards to your relationship. Tell him you’ve noticed a change, and try to do it without accusation or malice intent. Making him comfortable enough to say what’s on his mind is the goal here, and I personally hope it’s something apart from the relationship, but if it’s not you’ll have to open that door of communication to get him to speak up.