The beauty of iOS over Android is one of cohesiveness. Android seems "balkanized". Is something deployed guaranteed to work across all Android devices? I'm firmly in the iOS camp as an end user and my now-decreasing development efforts are on Linux, primarily Python and Bash (systems deployment and maintenance). Android, while nice on a Pixel (friends) seems almost broken on other devices.

This might be the case 7 years ago, but since Android Jetpack, a lot of Android development has been standardized. It used to be that it behaved differently across manufacturers with things like camera and file systems, but even this functions roughly the same these days, with CameraX and the newer media permissions.

Would you say it's better for him to learn Kotlin over Dart/Flutter? Seems to me that Flutter might be a better bet?

I don't know anything about Dart, though. I'm not keen on Flutter, for a number of reasons, namely some lower level things. The company I'm in took a long hard look at whether we'd do Flutter or native, and we went with native because it gave us more control and is more predictable.

Kotlin offers much of what Swift does, both try to adopt the same features as the other. It does pseudo-functional programming nicely, which is what a lot of people look for these days.

If you want to compare career paths for both, Dart/Flutter is still early and doesn't have enough data points. Kotlin's popularity grew at a similar rate early on, despite no MFAANG backing, and now Google is officially replacing their docs with it. The fact that Google pumps out so much recent effort into building up both native Android/Kotlin and Flutter/Dart means something, but on a gut feel they're investing more into Android/Kotlin/Jetpack.