The B+ is a fantastic place to start. It’s basically everything you need, minus the screen, to get started with programming and tinkering- which is good!
The version of Minecraft on the Raspberry Pi is very, very different from the one you might find on a laptop/desktop or tablet computer. It’s extremely cut down and geared towards helping kids learn how to program for and interact with 3-dimensional space.
People have used this, plus the Python-based programming interface and some out-of-the-box, to create really amazing things. There is, for example, code that will build blocks in Minecraft to match what a camera is seeing- creating a giant 2d photo in the game. And people have created scripts to build houses on the fly.
Designing an App for Android is a good start! You’ve honestly got me at a loss there, since I’ve never designed for the platform and I don’t know if the Pi would lend itself towards doing that. It would be a wholly different experience, but hopefully a rewarding one.
I would like to think the Pi would be beneficial either way, but I think its use would be contingent on him finding something to do with it- that’s the canonical problem with the Pi, people pick it up and don’t find something to do. But the easy retort is; if you don’t pick it up, you’ll never know, right!
A simple anecdote to support this; I picked up a Pi 2 years ago and it changed my whole career. It is what you make of it!
I’d suggest, if he’s got any sort of affinity towards designing or playing games ( other than Minecraft ), that the Pi could be a great way to get into game programming. In Python a lot of the guys I know use something called PyGame and I’ve seen 13-year-olds do amazing things in this… often to my total befuddlement since I’ve not used it much myself!
If, for example, he created something cool, there’s a whole community of people with Raspberry Pi computers that should run what he’s crated- it’s a great environment in which to share.
If the Pi turns out to be a non-starter on the programming front ( it really shouldn’t ) then you can always turn it into a home media player, a games console, a home server, or a variety of other useful purposes. And doing those things alone is educational!
Anyway, hopefully you’re now not so buried in enthusiasm that you can’t see your screen- let us know what you go for, and I’ll surely be around to help with any questions/problems/recommendations from there!