Any further decisions you make will have some effect, but they are very secondary to that big lump of principal that you're preserving.

You can <<probably>> turn that into 13.5K just by putting it into a 5% HYSA or money market account. In a similar situation with my daughter's shorter mission, I used Empower cash, since she had a shorter window for risk/reward. Those rates could drop with a rate cut, but probably not very much very soon.

I'd advise against a CD, only because sometimes missions can be abbreviated unexpectedly, and the rate benefit is not good enough to be worth the inflexible timeline in case college came a year earlier than he thought.

You can <<probably>> turn that into 15K+ with an index fund or 18K+ with selected individual stocks, which would be even better. But, as you know, that might also turn into 9.5K or 11K. If that "floor" still sounds like it would be good and welcome news to your son, then swing for the fences.</probably></probably>