I do not have a son, but my boyfriend is a Marine (has been for 8 years) and my co-worker's son is a Marine (has been for 6 years).
My boyfriend wanted to join because he always had a fascination and reverence for the military and wanted the honor of serving his country. He also liked the physical challenges the branch offered to him. His mom was supportive, but told him he needed to go to college first. Once he had an education and could make more of an informed decision, he could join, so he went to a military college and enlisted from there. He is now an Officer (Captain) and loves what he does.
My co-worker did not necessarily support the decision and to this day is not happy that he joined, but loves him dearly and is ecstatic every time he comes home to visit. He was 18 and didn't need permission from his parents. He was interested in joining because of the adventure and it was essentially a job where he could earn a pension after a while. He is a Sergeant who has been to the Middle East several times.
From what I've seen from both of these situations, it is important for parents to be involved in the decision, to make sure they are joining for the right reasons (not to run away from a situation at home, etc.) and understand what the decision entails (how long is the commitment, and so on). If you want to learn more, you may want to talk to someone who is a Marine rather than a recruiter because recruiters will tell you what you want to hear!