I have worked running programs for young adults and have worked with programs that help troubled young adults. First and foremost advice any of these programs give is "finish your education". He needs to get a college degree. At least a BA, if not a masters He needs to go talk to the guidance counselors at his college, lay it all out for them, and let them help him pick a course of study.
He will, most likely, not be able to obtain a civil job. But he could go talk to the local volunteer firefighters - they may take him on. He needs to realize that finding a job will be harder for him than for his peers. But, he will find one eventually. It may suck, but it will provide a, possibly nominal, paycheck and, most importantly, a work history. He needs to reach out to adults that he knows and see if any of them will provide him with an internship - doesn't have to be paid - just has to provide work experience and build that responsible citizen "card", and provide him a good reference.
He will have to prove, over and over again, that he is responsible and his conviction was a one off; a result of a youthful indiscretion. I know that in my state, one time offenders, with the offense being committed under a certain age, can, after a time, apply to have their record expunged. He needs to look into this.
He needs the encouragement of those around. He needs to be told to ignore and stay from anyone telling him to give up. Sorry, but true - anyone who would tell a struggling young adult that their life is over and worthless is, to me, a worthless person.
Most importantly, he needs to realize that he is so very young still. He has decades ahead of him and many people have overcome worse odds than his. Find some real life success stories for him to draw strength and courage from.
While he has stumbled on his path - his path, and his future, still stretch out before him.