It could be the tooth, if it has become reinfected and the toxins around the apex are leaking up the gum line then it could be that and he may or may not experience pain with it, but he would likely get a bad taste in his mouth from it.
This sounds really grim, but it’s what we used to suggest to clients in the chair: floss between two rear molars, and then smell it. If it’s a similar smell to what his breath is like, it could be just having a deep clean will sort it. Other possibilities is tongue, harbours loads of bacteria, so include brushing it in cleaning regime. Dehaydration can also cause bad breath. I would suggest if it is a new issue, its best to get it checked by a pro to ensure nothing that requires treatment is going on. Peace of mind really!