ITHURTs, though his name is first, is your name on the account? If not on the account directly has he given the cell phone company an ok to speak to you about the account? If so, and you have a suspicion about a particular number call the cell phone and ask them if that number has been called and how often has it been called in past couple of days.
Also, if you think he is in an A, now is the time to be try to lay low. If he is taking suspicious calls and he doesn't think that you know about it. Let him think your are clueless, unconcerned. Then, when he lays his cell down or when he is resting take down every number you can. Try not to be obvious. Then go back to cell company with these numbers and try to track them if possible. Tracing them is expensive. Some cell phone companies will trace the number for you if it is on your bill and it isn't familiar to you. You just call them and mention, this number isn't familiar. Would you do a reverse directory?
If this doesn't get the info, sometimes just laying low a couple of days and listening around you, you can find out who the number belongs to.
About if he is using a prepaid phone and forwarding calls. I'm not sure on this b/c phones can vary in there functions from one to the other. One phone may give the caller ID info and another may not. Again. If you have an idea the type (model and so on) of phone and company it comes from call them and ask them how the phone works on the caller id or better yet, go by the store it is from and even ask them about the model.