I was a semi-pro drummer for forty years, and while a primary TA coached Yr2 children on percussion to accompany the Christmas production for ten years. I also had recorder groups, and a Yr6 keyboard club for a few years.
What sort of music does he listen to, and what style does he hope to play? Has he done any other music at school, maybe recorder? Do they have percussion instruments in his school, and does he get a chance to play them? Knowing something about music, and the way it 'works' is necessary for a drummer to be able to accompany other musicians.
Doing ONLY drums is not really a good introduction to music, so I would suggest getting as good an electronic keyboard as you can afford - at least 61 full size keys, and connections for MIDI, a headphone socket, and ideally AUX inputs and outputs.
Children can learn keyboard from tutor books, which often come with a CD demonstrating the tunes, but it does need fairly serious concentration and effort. A keyboard will have several hundred sounds, many rhythms and accompaniment styles, and probably a few 'drum kits' which can be played 'live' from the appropriate keys, or programmed into memory.
To start drum technique practice, there are many tutor books, and I would suggest the easiest you can find as he is young. Get him a pair of fairly heavy sticks, and at first he can just learn the rhythms using a thick telephone directory as a 'practice pad'.
Depending where you live, there may possibly be Boys' Brigade, Salvation Army, or even samba bands, where you can get more information and possibly an opportunity to join in.