*Waves to Mod*. SGT USAF 1989-1997, Computer Communitcations Journeyman.
First off there is two different elements to the Military, the Officers and the Enlisted. Officers 'run things' and have all the degrees to start off with (in fact you can't be a Officer if you don't have a Bachelor's degree before your sent to Officer School, except in specialized circumstances - i..e Battlefield Commissions). Enlisted are the 'grunts' to 'do' the work ordered. They need the necessary training to complete that field, but they aren't going to be the 'Lawyers, Programmers, Doctors' you may want for your son.
If he WANTS to join the military, then he will, there is no 'sending him' as was in the case of the draft (i.e. gets drunk, has a bar fight, judge tells him he gets to serve his country instead of jail for 5 years for assault). If he sees the opportunity worth it, he will put the effort in as they say 'only those whom put themselves into something get something out of it'.
If he has a 'degree' in programming, then he could apply for OTS (Officer Training School) with a LOT better paycheck immediately, many more opportunities, and he can 'boss people around' (considering his aggressive attitude your saying that might all appeal to him). If he just 'programs' then you really have no clue and I wouldn't go focusing on that, he can be appointed for many a task.
Army: Pretty much all soldiers are expected to fight, and are trained how to fight in combat, then escalated to a specialization based on ARMY needs. Ranks for Army Enlisted are nicknamed 'Velcro' because of how quickly they hand them out and strip them off the shoulder. Officers are the 'Elite' and expect to make a career out of it, but we are taking very diverse fields from Nuclear Physics to commanding a Platoon of Tanks. Some are very career forward, some are very dead end, but they are stepping stones for Officers to 'move up' the political chain. Enlisted get screwed because if your a Tank Driver / Gun Loader there isn't any 'Civilian' jobs similiar to that.
Navy: Again split, the Officers command the Enlisted, but both Officer and Enlisted is expected to learn / be able to function in ANY part of a SHIP. That means they can be clearing out the Bilge (toilets) to hanging on the side of the ship during a storm repairing the electrical communications gear. The Officers are meant to earn thier way up to 'Command thier own ship one day', while enlisted deal with alot of 'crap duty' till they get higher enough they can order the 'new seamen' what to do.
Air Force: Often referred to as the 'Desk Force' since we all revolved around a Air Force Base to be on, to get the planes up to do the 'job' they were designed for. Highly technical, but all Officers (like Navy) are expected and trained to fly aircraft (primary job), and work the 'political trail' from job to job to move up the 'right chain' to get the better jobs (i.e. being in charge of a Air Force Base of thier own say in Hawaii?). Enlisted are expected to do the work, but be highly technically skilled, not necessarily 'educated' as the civilian counterparts (is why the Air Force has ALOT of Civlian / Contract workers to 'reduce costs'). All Air Force are now expected to be able to 'military ready' which means trained to pack up and deploy within a few hours to any place in the world, setup a new base, and to 'augment' the other forces (remember all those convoys getting blown up on youtube in Iraq? Those were Navy/Air Force people doing the job so the Army/Marines could to the 'real fighting').
Marines: All Officers and Enlisted are MARINES, period. While the Officers command and have similiar jobs like the Navy they fall under (they aren't a separate 'branch') like Attorney's, Doctors, etc. while the Enlisted does the 'labor work', all Marines are expected, trained, and under the doctrine to FIGHT as Marines, first in, last out, with thier hands to any modern weapon. So a Officer would be standing with the Gunery Sgt telling the other Marines to fill sand bags, but then grab a shovel and fill them as well 'fortify the position', grab thier gun and be part of the combat, while directing and managing the 'troops attacks' through the Gunny. They are very unique, hardcore, and exemplify the demands met upon them.
Some source material: ' Saving Private Ryan' (WWII Soldiers), 'We were Soldiers' (Vietnam Soldiers), 'Black Hawk Down' (Iraq-Generation Soldiers).