I went to college straight out of H.S and like many others did a lot of dumb and risky things. School was secondary to just having a good time. I enlisted into the Air National Guard and came back home refocused and went on the earn academic honors each semester until I graduated.
Not only did it help put me back on the right track but I now am a full timer in the Guard and love it!
In the end this may have been the preferred path for him if he acted out a bit during his high school years. Potentially him going to college with ZERO supervision would have proved to be more problems. Atleast this way he will be held to accountability and learn leadership skills that he can use outside of the service.
For now attempt to put your fears and worry away and just enjoy the graduation. Give him a big hug and let him know how proud you are of him. What happened in the past is just that....the past. He's done something that only a small fraction of the population ever do in serving. He's off to a good start in his adult life. Here's to hoping he continues to keep impressing you.