My son is 7 years old and is still wetting the bed most nights. I have tried the usual things (restricting drinks in the evening, 2 or 3 trips to the loo before settling down etc) but nothing seems to work. I have thought about an alarm as the problem seems to be that the urge to wee just doesn't wake him, trouble is, he shares a room with his four year old brother and I am worried that it may wake him too. Any advice/ideas would be most appreciated.

Pullups will help to relieve the stress of expectation that he should be dry. It is a sign of stress, and the expectation puts even more stress on him.
Then look for stress in his life and find ways to help him express his emotions freely so he doesn't have to let it out in his sleep. There is a section on this in Raising Our Children, Raising Ourselves (yes, I am nuts about that book) that can help you a lot.