This is a sign of withdrawal. Think about this sign carefully and do not be too quick to think there is a problem. Your child may lose interest in some people and some activities but this usually has nothing to do with withdrawal. Perhaps he became bored or simply got interested in other things. This sign is more than simply changing his interests.
Watch to see if the people and activities in which he is losing interest are being replaced by other people and activities. If so, a problem is unlikely. If not, your child may be withdrawing. Talk with him about what you have noticed. You can say, "I've been noticing you're withdrawing from people and activities. It isn't just changing interests. For example, (Give some examples.) you stay by yourself and don't seem interested in much. Will you help me understand what's happening?"
He may tell you he is into his school work and does not have time for activities. If so, say, "School work is okay but a steady diet of it is too much. I think there is more happening. Talk to me about what happened with your friends and with the activities you had enjoyed." If you are patient and give it a little time, he will likely tell you what happened in his world to cause things to go sour.