Anyone can own a RIF.
However you can only buy a RIF if you have defence to own one. That been UKARA membership in the case of airsoft guns, Or be part of a society that is recognised by the seller. The whole system is designed to protect the seller not the owner. So the seller can prove to a court they took every precaution to make sure they sold a fully black gun to a responsible owner.
2 tone guns can be bought by anyone without defence, but the gun has to be majority bright coloured.
In public however things are very different. You can not take a RIF or IF (2 tone) out in public. The law in all instances can and will treat them as firearms, because anything that looks like a firearm - is a firearm and the law will prosecute as a firearm. So you can't walk about with one in public, you can't play with them in a park, and if the neighbours can see and are alarmed and call the police even the back garden may be dodgy, Especially if it's been aimed at the neighbours houses.
So it's not unlike air rifle ownership, where you always keep them in a case, slip or bag until you get to the place you intend to shoot.
So back to your grandson. If he's old enough to play airsoft then think about taking him. It might cost some money to get the gun sorted as it may need a service and chrono to make sure it's at the legal and site limits. A lot of airsoft site do under 18's games and they can be fun. It very much might be a case of doing a day with the expense of hiring a gun, but taking yours down as well and getting them to give it the chrono and once over just to be sure. Typically a day is about £40-50 each with hires. After 3 games over at least 2 months you will be eligible for UKARA membership.
There are a couple of sites local to Wakefield. Personally I prefer outdoor sites but indoor can be fun as well. A simple google search will throw up the sites in your area.
This one might be worth a look at if all of the above makes sense.
Get outdoors with Yorkshire tactical activities, Airsoft, Paintball & Laser-tag sites located in Wakefield, Sheffield, Barnsley, Bradford & Doncaster.
www.yorkshiretacticalactivities.co.uk