They'll accept a GED and under certain circumstances, an arrest record. But if he fails a piss test at any time they'll give him the boot. Might want to inform him of that.

Which branch depends on his aptitude. If he's mechanically inclined then the USN has the most jobs in that area. If electronic then it used to be the purvue of the USAF & Navy but all branches are going electronic. If he's interested in law enforcement and working with the public on small craft, nothing beats the Coast Guard. Like I said, it all depends on his aptitude. None of the branches appreciates a slacker. All of them believe in employing a wide array of motivational methods for those few folks who seem less willing to participate in group activities.
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In my case I have encouraged my kids to consider the military. One is marrying a soldier -- being a spouse is a military career. The other two are have considered it but thus far not expressed an interest (one of them shouldn't even consider it, she's a bright kid but couldn't give a command to a dog without hugging it first).

FYI I would not encourage a young woman to enlist, though I would a young man. I don't think MOST women can hack it and thus, become a drag and/or source of dissension in the unit. Female officers have a different world to inhabit and in that world, generally perform equally to male officers.