How are the ecosystems? Does the choice of brand lock you in or out of particular titles? I suspect his main goal is to play gorilla tag and similar.

The Quest has its own ecosystem of native VR games that run directly on the device. You need to create a Meta account to use the headset. However, it's possible to sideload apps to it and you can stream PCVR games to the Quest.

Is Metaquest 3 an objectively good/decent product? What are some viable alternatives?

As of right now, the Quest 3 is a decent-choice hardware-wise, probably the best in terms of pure value and it has the largest userbase. The big downside is that it's made by Meta. It's also a bit of an unfortunate time to buy a VR-headset, as it looks like the upcoming Steam Frame will be a better choice, once it releases. If your kids birthday is still a few months away, it might be worth waiting for a bit before buying anything. It's completely unknown when the Steam Frame will release though.

Something like a Pico Neo 3 could potentially be an alternative if you want to avoid Meta. For purely PCVR, there is also the Valve Index, but that's more of an enthusiast-level device that requires a more complex setup using base stations for tracking.

He’s got a decent phone and a half-decent PC. Will the same headset work for both?

The PC should work. It doesn't stream phone games.

How much HPU (if any) is required to drive one of these decently?

GPU I assume? Well, for one thing, it can run games standalone, though those games won't have impressive graphics. For PCVR streaming, I'd recommend at least something like a 1070, preferrably something much better.