The pain is not much of an issue, maybe let him play indoors and cqb rather than outdoors. Just feels like a sting, from time to time you get a small bruise that's the size of a pea, but it usually doesn't get much worse than that.
The issue is safety, a 10 yr old with a gun in the house which shoots 100m/s+ BBs can go downhill quick.
You can easily go blind, and I'm not sure how mature your boy is, but I'd keep the gun locked and teach him gun safety properly.
A bunch of 10 yr olds playing together can also get them to be less lenient on safety, even if your son is well-behaved, just keep an eye over them.
Please teach your son that it's not something to show off, or be outwardly proud of and tell everyone around him about. Airsoft can be culturally unacceptable to some people and it's best not to mention it, especially since he's only 10 years old.
Full face mask is good, but for children, it can get heavy and bulky. I think low profile full face masks are better, not massive rental masks.
get him some padded gloves too, don't want BBs hitting the bony parts of the body.
In terms of guns, you have to get him a 2-tone unless you have a UKARA membership.
When it's 2-tone, it doesn't look very formidable at all. but of course, if you're looking for something that doesn't look too real, then get him something like this:Specna Arms - SA-F20 FLEX™ Compact Carbine AEG | Patrol Base UK. You can spend more money, I don't know how much you're thinking of spending. I recommend NOT buying any other brands than the following: Specna Arms(Flex and Edge series), G&G, Lancer tactical(Gen3, Gen4, don't buy Gen2 or earlier generations), Arcturus(expensive). These brands are very standard, won't disappoint or break.
Try to get a gun with a mosfet in it if possible, not needed but makes everything better. After buying it, the first upgrade you should do to it, is changing the bucking (hop up rubber) to something like Unicorn AEG buckings. The only accessory you really need, if any, would be an optic, like the £30 ones from Amazon or smth.
Just make sure the gun doesn't weigh too much or is super long, running around with a loadout for hours is tiring for anyone, let alone a 10 yr old boy.
Get AEG's for him, not GBBR, GBB or HPA. Just makes things easier to manage for beginners.
Airsoft guns can shoot a lot further with less down curve than most paintball guns. 30m for a stock gun and up to 100m+ for an upgraded build. It's quite different to paintball in my opinion; the way the games are played, especially outdoors.
I'd recommend going to a friendly field, like mayhem airsoft, and let him try a few games with a rental gun before going full in.
But beware that rental guns are usually shit, so he might not have a good time compared to using a proper, new decent priced gun.
These are just my suggestions, some of them are objectively true, some are just preferences. I'm sure other people have something to say as well.