My son never seems to do any work, he says he gets more done at his girl friend's house, but I don't know that he actually does any work there. Should I stop him seeing her until after the exams?
You may be right to be worried as he might just be avoiding the work and be distracted by more appealing company, or he may have a point- if working with her is more productive than pining for her alone then stick with it, although try to encourage a balance of some time alone and some together as they won't be doing exactly the same subjects or have identical needs in each subject. When he is working at her house you need to find out if he has used his time productively. Asking to see evidence of the work might work but it could lead to confrontations or him just showing you some old school work and not all revision actually produces something to look at. Ringing the other parents might be a good idea and sharing your concerns; they are in the same boat as you and might be able to reassure you. Above all else treat them both as adults and if needed sit down together as two families and work out a strategy. In a similar way studying in groups with friends might help, especially from the moral support point of view, but obviously the distractions may win so don't base all the revision on this one method, but use it in conjunction with a variety of methods.