Do you know – or have you asked – whether he wants to be a mechanic in any capacity, whether for the Marines or somewhere in the private sector? If yes and he’s in a bad job situation, why not try switching jobs to see if there’s a work situation/boss that works better for him? There seems to be enough demand right now that depending on where he lives, he should be able to get something. He doesn’t have to do this to the exclusion of the military. He can switch jobs, hopefully be happier in his day to day life, and continue to talk to recruiters to figure out which branch works. When he’s happier, it’s likely the decision will be made with a clearer mind – i.e. going to the military bc he wants to be in the military, not because he thinks it’s his only option to get out of a bad job situation.
He’s only 20 – is there some other trade or job that he’s interested in? Is there a way he could try something else – like working for a contractor/learning to be an electrician etc? Sometimes there can be some satisfaction in a trade where you are building something and can point to a finished product and say – I built that house.