I am not a Marine, although over the decades I've known quite a few, and became friends with several. This includes guys who fought in every conflict from WW2 to the current conflict, and I play poker with a revolving group of young marines.
In my experience there is a significant maturity gap between 21 year old civilian coasting through community college, a 21 year old E3 in the Army, Navy, or Air Force,and 21 year old E3 Marine. It, sounds cliche but Marines boot camp does turn boys into men.
It sounds like your son wants to join for the traditional reasons, challenge, adventure,and desire to serve the country. Frankly exactly the type of young men the Marines are looking for.
I highly recommend that both you and your son read this book
Making the Corp, which is about boot camp, and Marine corp life and traditions. The book is written by Thomas Rick, long time military/war correspondent for the Wall St Journal and latter the Washington Post. He is good writer and quite a few parents facing exactly the dilemma you are going through highly recommended on Amazon. It is interesting and easy read, although since the bulk of it was written in 1997 a bit outdated. I also think you will gain a profound respect for the process the Marine uses to transform kids into Marines.
I had a chat with Nords about kids entering the service recently. One thing is clear as other have said, from a career prospect, civilian employment, danger etc. the other services are better choices.
Marines are the less likely service to put in the 20 years needed to collect the military pension. Generally the Marine scores the lowest for quality of life and generally happiness of all services. Finally, the chance of your son being killed or in injured on the jobs are roughly 25% higher in the Marine corp and 3x more than the air force. Although it is worth noting that overall mortality rate of the armed service is lower than the civilian at the same age. Evidently the benefits of being in shape, good medical care, outweigh the risks of being shot at.