That's pretty awesome. We have a mountain bike we bought for my nephew that we keep at the house so when he goes camping with us he has something to ride around the campground. He seems to enjoy it. This looks like a great hobby for your nephew. It's great he is learning to adapt and excel.
He does a lot of things actually, only tonight I saw a video of him playing the Top Gun theme (from the movie) he'd taught himself to play on his violin...at five and a half years old! Lol. It was pretty terrible...but one could make out the song; he wants to be a fighter pilot so loves Top Gun.
He's doing en enduro in a couple weeks, on his bike and a few other things are coming up. He has a diverse set of interests like drone flying, Lego, drawing, biking, the beach (his mum and dad go kite boarding there all the time), kayaking with mum and dad, likes off-roading, camping and...well, he does all sorts of stuff. It's cool.
But yep, as you say...a kid with a bike is often a happy kid.
That is awesome!