I ride/race 125's in the woods (Vet class), and have found they're a great combination of sharp handling, snappy motors, and less tiring than other choices. Not that you asked, but here's what I always recommend to other 125ers headed into the bush:
Drop a gear on the front sprocket. This $8.00 move will get a better result than a $200 pipe. You'll never see 5th gear in the bush, anyway. While you're at it, toss an O-ring chain on and keep the stock as a spare.
Boyesen reeds help with low end response and give the snap you need for clearing logs and roots. They don't increase horsepower, but can "wake up" a lazy bottom end.
You'll love the flywheel weight and wonder why they don't come like that stock! Easier starts, difficult to stall unnecessarily, hooks up with mother earth instead of spinning everywhere. Increase your idle a bit to allow the engine to stay running while you're in gear with the clutch in.
Re-route non "draining" carb hoses up higher, or ideally into the airbox. Radiator guard, skid plate, hand protectors. If you're handy, you can make a grab-strap out of an old tie-down and mount it between the seat and rear fender. When the stock bars bend (about next week), consider a shorter bar length to slip between trees easier.
Truthfully, dropping a gear on the front and getting your jetting crisp (especially airscrew for that 0 to 1/4 throttle response) is going to yield your best results short of getting a big-bore kit or port job for low-end. The aftermarket pipe idea isn't bad, it's just expensive. Stock pipes tend to be more durable and quieter. Save your money for the best safety gear you can get for your child. For the same cash, look into some helmet communicators
http://www.collett.mb.ca/ My buddy and I have had ours for a year, and can't imagine long rides without them. They add a new dimension of safety and fun to riding. I hate riding with people who don't have one!
Your son is very blessed to have a dad who wants to ride with him. Happy trails!