Without a high school diploma, he will have a difficult time supporting a wife and child. Enlist the help of a military recruiter to talk to your son about this reality since he is not listening to you. The military is more concerned about education levels than in the past. The Armed Forces will want your son to take an ASVAB test if he has not already done so. Those results could possibility limit his schooling and job opportunities in the military.
Stick to your guns and do not let him leave school. Let him know that if he fails on purpose, he will likely ruin his chances of getting into the military. Not graduating will likely guarantee him a minimum-wage job in the real world.
He has engaged in some very grown-up behaviors that will force him to grow up faster than if he was to pursue a college education. If you co-sign for an apartment or car loan, be prepared to pick up the tab if your son cannot pay his rent or payments. This would not be good for him or for you.