My 18 year old son broke down crying because his girlfriend of 3 years left him. What can I do to help him?

Tell him it's very natural to feel hurt and that although it may not seem like it now, it will pass. Then suggest he head over to the forums at rsdnation.com , which have lots of men's advice regarding dealing with women and relationships, particularly learning to be good with women in general, rather than obsessing over one particular woman.