Kingston Wheelers has quite an active CX and Gravel scene and is a friendly inclusive club with quite a wide mix of ability. Lots of social group rides and events as well as rides for the more serious and fitter/competitive riders. They usually organise new joiner/taster rides on the first Sunday of each month.
They also have a GoRide club attached which may be of interest to your son. It runs on a Saturday morning at the YMCA Hawker Centre, under 8’s from 10.30 and 8’s upwards from 11.30 (some children between 6 and 8 also join the later group depending on ability). Many of the parents also ride with the Wheelers and the two main coaches are both Wheelers. (disclaimer, I may be one of those coaches)
Twickenham Cycling club also are another friendly local club and London Dynamo is another club that also operates in the Kingston/Richmond area, they generally pursue a more competitive ethos.
There are numerous other smaller clubs in the area all of which like the above have their pros and cons.