What could be up with him?

My generally sweet friend (and ex boyfriend) has had some very obvious mood and behavioral changes. He's normally sweet, friendly, and loving (always quick to comfort me or one of his other friends when we're depressed) but has suddenly become very hostile. He's screaming at people and using profanities excessively. He had a family member die over Christmas break, and was a bit depressed, but last week he suddenly started acting strangely. He's secretive and hostile and withdrawn. What could be wrong with him? I'm worried and the things he's said are really upsetting me. We suspect that he's on drugs.