No, he doesn't have to decide it it's a career. He hasn't even started on his first boat yet. If he likes it, he should stay in. If not, find something else to do, and with his clearance and training, he'll have a ton of options. There are lots of folks getting out after 3 or 4 years, some after 15 years, and some stay until or beyond 20.
The pension system has changed too, so now there is a bit less financial incentive to stay until 20. You can read about it here if you want:
https://militarypay.defense.gov/BlendedRetirement/
ugeaux girl, I would highly encourage that he max out the TSP, at least up to the full matching amount.