I think he's seeing someone else. If he's not sleeping with someone on occasion he's thinking about it. There are clues, changes in his habits that prove it. Habits in his personal grooming and hygiene as well as changes in his way of thinking. I started to pick up on it about 6 months ago I think.
He has been shaving his pubic. In 15 years he never shaved for me. He also stopped biting his nails... Or at least he managed to stop ganging up on one nail. One of his nails was almost gone due to him picking at it so much. It was disgusting really. I don't know when but he's managed to get it looking like the rest.
Those are two of the more obvious things. There are other minor indicators around his personality and habits that ring warning bells: liking different types of music, downloading stuff he'd never downloaded before like international stuff rather than just local stuff.
I'm really tempted to outright ask him if he's sleeping with someone else to see his reaction. But I'm not sure it's the best course of action. Any suggestions?