Sound like he is going to grow 5 or 6 inches. While you can get a smaller bike and later raise the seat, if he really gets into biking he will want a larger bike that fits properly later. If you buy a larger bike now and lower the seat to the lowest setting, he won't enjoy riding it and will never get into biking.

Tons and tons of great used bikes on eBay or, even better, locally on craigslist or Facebook Marketplace so he can test ride the bike. I bought a virtually new (100 miles only) carbon Jamis road bike that lists for $3K with some added options worth a couple hundred for just $800. It is the bike I'll ride for the rest of my life, just upgrading parts as needed.

Or ride trial bikes at local bike shops and maybe buy one from them, again used, last year's model.

These are all great brands for road bikes and some cross bikes (I don't know dirt bikes). The biggest: Specialized, Cannondale, Trek and Giant. Smaller brands just as good: Jamis (mine), Felt, GT, Orbea, Look. I'm not mentioning crazy expensive bikes like Cervello, Colnago, Pinarello. If he likes biking and has money to burn at 25 years old, he can buy those himself.

Components matter. Every component makes has several levels. You don't need the most expensive (like Dura-Ace or Ultegra by Shimano), but don't buy the cheaper plastic sets they make. I have the mid-kevel Shimano 105 and it is a solid long-term workhorse set. No shortcuts on checking the components. You'll likely need an honest bike shop to help you figure that part out.

Having him try out bikes for a while will make him feel like he's involved and invested. That's when he'll get into it more. Good luck.