The boy needs a new bike, might get frame and forks, but what forks?
For what purpose and what wheel size?
At that weight my son was on a pair of old custom valved SIDs
SId would be my most obvious go to thought too. High end, but older, XC forks, will be plenty stiff enough but tunable enough to work at his weight.
Not one with a Motion Control Damper.
Nothing hardcore, just a bit of XC kinda thing.
27.5"
Not fox. Your boy is too light and way below the minimum operating wight for them. My wife is only 50kg and cant use them, they wont deliver full travel with 50% sag. Rockshox work for her and can be set up to use full travel at a sensible sag
I'm really light for an adult man of my height, 10st4, and I've always found Manitou to be be best for lighter riders (and a lot more reliable than Rockshox!)
However, they are American so I'm not buying a new set again. Maybe a second hand set of Manitous? Which type will depend on wheel size and travel requirements,.maybe some Machetes for 27.5 and cheapish, get the Pro damper
I'd be looking for a set of old dual air RS's so you can fudge with negative pressures for sensitivity. Worked a treat for my son's previous previous bike.
I had some Rebas/Recons (32mm) dealt with by TFTuned which included low friction seals. They worked well on daughter's bike who was around 38kg.
Not cheap, mind. Was 145 quid for service, seals and travel adjust, but they worked way better than just letting air out.
I've also had mantious and like mentioned above, they seemed quite good at lower pressures.