A couple threads exist on the topic. Here's one, but I think there was a bigger one at some point:
https://texags.com/forums/63/topics/2608930Buy something completely unrelated to the military that is just nice. Or buy something that he'll use for a long time in the military. Personally, buying Gold O-1 bars wouldn't be my go to as he'll wear it for 2 years, could get lost quite easily, but would be useful as an investment considering gold prices.
If there's a nice uniform item that's option, and too expensive for him to buy, then maybe splurge for that. Or perhaps help him upgrade to the company that makes the nicer version of the uniform.
A nice watch that he can wear in uniform with the date of his commissioning and "from mom/dad" engraved on the back would also be nice.