I had the KFX50/90's for my kids.  Solid, good, basic quads.  They are actually built by Kymco and branded as a Kawasaki.  You can get parts for them easily, and they will hold their value mostly.  The Suzuki lt50 is a 2 stroke and harder to find clean ones.  If your trying to stay 2 stroke you would then need to find the LT80.  Again it wakes up quite a bit with a pipe.  They make better power and a simple pipe will up the power when your son gets used to the stock one.

My kids also rode out at Glamis and Buttercup on those small things.  They could follow some group rides, but they cant climb worth a crap.  After the 50/90, I bought both my kids Honda TRX250EX's.  Great to learn the clutch on as they are shifter quads with sport clutches.  Again they hold their value pretty good.

I would recommend NOT making many modifications to the 50/90 quads as they hurt resale and just don't gain that much in power (2-strokes are the exception).  Plus your son will out grow the 50 and 90 quickly.

Good luck
Larry

Is that too young for a 250EX?

I'd recommend a Apex or DRR. They can go from 50 to 70cc (when ready).
They have suspensions that actually work.
They will give you years of service.