You didn't say if he wanted to make money or do something he likes. If he wants to make money he should rob a bank, become a lawyer, an orthodontist or a politician (did i miss any?) but probably forget engineering.
But if he wants to make a reasonable living and enjoy doing something he likes, that's something else.
Tell me, does the USA still give GI Education grants to its ex-servicemen? Wise use of that will help. I know my father used his after WWII. A curious fact is that while the US Army invested 13 weeks training him as a cook it took only 8 weeks training for the parachute infantry (what was later the 509th)I expect, would hope, it was different for your son and the educational opportunities will be much better now.
Instrumentation (my own niche)has been mentioned. But beware, there are lots of guys with instrument and lots of guys who need them and they never meet up. It gives you an edge if you really know the industry you're going to serve, whatever you decide later is your niche. So perhaps a college degree then start work in the industry, move a round a lot to get exposure to different aspects of the business and look for the main chance. It might be instrumentation, it might be systems supply. Find a suitable niche and he could aim at seting up his own business.