I think it would be great experience that he won't get otherwise. Plus the additional connections with others he will make. With his current GPA and your observation that he is not taxed, I suspect he is disciplined with his time as needed and focused on what he needs to be when he needs to be. I just finished a doctorate and all throughout my educational career I worked a full-time job while attending school full-time. During my masters I carried a 3.87, while also balancing pastoring a church in addition to a full-time job and full-time course load. It is about discipline, focus, determination, and lots of caffeine and prayers.
Whatever he chooses, it sounds like he is a high achiever. Congrats on that.