A bit of an aside, but how hard is it to get into building RC aeroplanes, compared to FPV copter drones?
That being said, I enjoyed every moment flying my planes. I built and flew quite a few quadcopters but they never felt that free because there's always that control algorithm between the pilot and the motors, while aeroplanes are basically just mapping the movement of the joystick to the servos. I believe the UK has a lot of great local clubs, and I believe that's the best place to get started.
Side note, when your son gets more experience in the field, he might wanna build his own gas turbine to power his planes. And this association based in UK is the best on this planet: https://www.gtba.co.uk
For UK delivery, let me look into how to set up international shipping. Will get back to you by end of the day.
http://wrightbrothersrc.com/products/gambler.htm
If the goal is the flying. You can't go wrong with an easy star. I've crashed mine a million times. You just patch it back together humpty dumpty style with thick CA + accelerant. Bonus points for the prop being in the back, so if you run into stuff you (probably) won't draw blood.
https://mrmpxhobbies.com/product/rr-easystar-3/
Note that the hobby does require some skill w/ flying and need some level of risk management. There are cords that let you plug your transmitter into a computer/fly over a simulation that can help with the former.
I haven't built a balsa wood plane in ages. But so, the glue of choice has changed ? No more balsa wood glue with atrocious fumes ?
The main difference in building planes is you have to pay attention to center-of-gravity much more; minute differences will make the difference between your plane flying amazingly, like a brick (nose heavy), or not at all (tail heavy). There's also more work to do in setting control linkages and surface throws. But, overall, it's not too tough with most models.
Takeoff with planes can be very stressful the first few times; you have to choose between ground/runway takeoff, which typically results in a very inefficient model due to landing gear drag and is prone to flipping over, throwing the plane by hand, which requires practice and can be quite hazardous with a "pusher" style plane with the prop at the back, and building some kind of bungee launcher, which you then have to set up and lug around.
Then you have to decide how to fly - line of sight or FPV. Line of sight flying is quite an acquired skill and has a very steep learning curve - you basically have to learn to "become the plane" and understand how your control stick inputs are affecting the attitude of the plane without being able to see it very well.
FPV plane flying, while less popular than LOS, is very easy and much more rewarding IMO. The reaction time in all but the most extreme plane stunt flying is much less dramatic than in FPV quads.
And, due to quirks of the general hobby flight control software scene, most hobby FPV planes have a working loiter-in-a-circle setting while most FPV quads have a barely-functional GPS rescue mode and little to no ability to actually hover (it's very rare for an FPV quad to "just stay put"; this is the realm of camera drones).
I fly FPV quads when I need a focus/adrenalin boost and FPV planes when I just want to relax and chill. You can fly planes in an adrenalin style, but they're much more conducive to just looking at the scenery and goofing around. Massive bonus points that most plane builds are almost silent compared to an FPV quad so you don't worry about bothering people so much.
Flying is pretty different, though. If you're used to a copter that will just stay put when you release the controls, flying planes will be an adjustment.