If you want something that he can do more hands on I'd suggest a
Kyosho Sand Master kit.. Okay so it's not a comp racer but it's a great little kit with some of the parts factory assembled so he can do a lot more for himself with supervision etc. It's something he could do almost all by himself while the two of you shared making something a bit more racy.
At only £80 the Sand Master is great value for a first hobby grade car and has a few hop ups on the way.
At the same time as he is tackling the build of his own car you could maybe share the build of a Tamiya DT-02 chassis based buggy... They are a little more complex and include diff builds etc but can be upgraded to be relatively competitive. They are a dream to drive for a newb and a lot of fun. He may not win a race but he'll have a lot of fun driving. And again at only £80 for a
holiday buggy you can't loose. They are almost indestructible too with a bit of commonsense driving.
For your £250 you can get 2 cars, all the RC gear and up grades and make the whole experience last longer and be more rewarding. You could probably get another DT-02 within your budget and start a new class at the track and race with your son at his level
K