I don't think there is anything necessarily wrong with the setup as is. If your son likes the setup and it fulfills the usage requirements, I don't see any pressing need to replace it as long as you trust the reliability of the light (I personally only use Surefire atm).
Could it be a bit less bulky and more streamlined? Sure. Optimally, I would say run an M300C in a Gear Sector offset mount at 11 o'clock using thumb activation of the clicky switch. With the Gear Sector mount you'd still have the option to trigger it from the opposite side. I like to avoid pressure switches and wires, but that is just my preference. I also run almost all my lights on the slung side because I like to use thumb activation; the M300 in a GS mount is so compact and lightweight I don't even notice it's there until I need it. So, an M300C would definitely shave some weight and be a lot more compact if that's the goal.
If I were to run a pressure switch, the SF SR07 is pretty damn nice. I especially like it offers momentary and constant on in the same location and small package, plus a built in rail mount. In that vein, another great option would be an SF M600U with a pressure switch, using either the factory mount at 3 o'clock or offset at 1 o'clock. You couldn't really go wrong with either of those, it just depends on what you/your son prefer and the primary use of the rifle.
I think rechargeable is perfectly fine as long as you understand their requirements and limitations (keeping the batteries freshly topped off in the charger; knowing that long term storage isn't really practical, etc.). They aren't applicable for every situation, but they are certainly viable, not to mention cost effective, as long as they are used properly.
To summarize, if you trust the reliability of the light and your son finds the setup comfortable, I wouldn't say you absolutely need to change it unless it just doesn't meet the usage requirements. Of course, I'm sure he wouldn't mind a shiny new M300 or M600, either :D.