I think the best way to know what he is thinking and why he is acting funny is to have a talk with him. You should approach him and ask to speak in private. Maintain a cool and calm tone during this conversation. You don’t want him to feel like you are trying to start a fight.

Begin by pointing out that you have noticed he has been distant towards you and be prepared to give him example of when he has done this because he will likely ask.  Let him know how you felt those when he was acting this distant way and why his distance is becoming a concern for you. Remind him that you care about him and just want for the two of you to be the best couple that you could possibly be.