I remember when my best friend finally returned from serving in Operation Desert Storm in the 90's. We were discussing how his family and I were worried from time to time with the particular dangers the Marines were facing in that campaign. I still recalled how he looked at me a little askance and said, "Danger? We weren't in any danger."
He was serious. He went on to detail some of the POW's they were interrogating had been briefed by their superiors that if they were to go up against or be captured by the Marines, they could anticipate the most horrible demise; a Marine "mythology" we Americans know as historical fact spun into an almost mythical reputation of ferocity by these Iraqi commanders to "prepare" their men.
Of course these superiors were trying to instill some type of will to fight in their men, but it ended up backfiring because once the Iraqis made contact with Marine units, they simply gave up. Part and parcel of the massive surrenders during the war. They were scared to death of the Marines. Fascinating, really.
Point being, your son, perhaps in your mind still that little kid you raised, now rolling with his buds, are all together some very, very scary dudes whose reputation precedes them globally, thanks to every other Marine who has gone before them. The deterrence they represent in the international arena is invaluable.
I know you are going to fret anyway, but when your son says don't worry about him, he's certainly not being whimsical. 
"Hell these are Marines. Men like them held Guadalcanal and took Iwo Jima, Baghdad ain't shit." - Major General John F. Kelly to a reporter who asked him if he ever contemplated defeat
