Emily,
Have him kneel on the floor, as this will get his body in a position that will take some pressure of his muscles down there. Then have him concentrate hard on those muscles which are now trained to HOLD it all in. Once he has a mental feel of those muscles tell him to try and relax them and just let all that pressure release into his diap. Sometimes it helps to (as a guy) aim your pelvis downward like you'd be peeing onto floor as it also relieves some of the pressure on those muscles down there which are being clenched tightly. It'll take a little bit but just remember the more pressure he places on that area the harder it will be to go.
Hope this helps
cwick