Look at the changed his behavior that was not so. What changes: look, words, actions, mannerisms, facial expressions. In what situation and how it changes his behavior. What and how he reacts.
Rate how much he distanced himself from you. What remained the same, as he talks to you, what is wrong in tone and where at the time of the conversation most often focused his gaze. All this will help you better assess the situation, to understand what is wrong and what to do in the next.
Analyze the main causes of his unusual behavior, try to figure out what was the reason that caused such unusual behavior of your partner. Maybe he's just used to you and began a period of "grinding"? Maybe you're just tired from each other? Or possible that your behavior something pushes him, forcing him to behave that way.
Understand yourself. Analyze their behavior, try to determine what exactly has changed in your behavior, try to identify those moments in which you yourself excluded partner, or trying very much to control it. Change your line of behavior, look at things easier. After that, if your partner does not change.
Talk openly with him, find out what the problems are, what became of your relationship and what's going on in his soul. If your young person would make contact, then he, too, noticed the change and wants to take it all back. If he is surprised and does not want to solve the problem it means that your relationship has outlived its usefulness, it's time to put an end. In the case when the partner tries mutual efforts to solve the problem, do not forget that a relationship is a couple's game in which the team consists of two people.