It's a great bike, but the maintenance requirements are significant, it is a race bike.
It can be made into a very respectable off-road bike, but be ready to spend quite a few dollars on it to make that happen.
The sort of off-road bike it can be though is one for more advanced level riders, to be sure.
The valves issue is not that they bugger up, it's that they are very high performance bikes with high power output,high rpms,high compression,and high heat. Therefore the top ends of the motors do require a maintenance schedule that must be followed .
A 150F or 230F are great bikes with outstanding re-sale values for when he's ready for something faster.
When the boy wants a little more, higher performance can be had fairly reasonably with jetting/airbox/exhaust/camshaft modifications which don't negatively effect the bikes dependability at all.
Just look into the BBR section on the Crf's Only on-line store

Doug