Sure you will get some answers specific to Kegs soon but in general they have to follow NC for games/PE the same as everything else so choice has to be very limited.
This was a while ago now, but at CRGS only those boys who were considerd so bad at Rugby it was dangerous for them to be playing were sent off to play football
Also a few years ago now - they had an OFSTED inspection that criticised the emphasis on Rugby to the exclusion of football so had to introduce more football.
What usually happens is there will be different games in different terms - would imagine rugby will be the Autumn term as this is when inter-school matches are played. Might get to play football in the Spring term or possibly during PE lessons. Not sure if KEGS do hockey?
Probably cricket in the summer term & athletics in PE sessions.
Post KS3 he might get more choice
Re rugby shirts etc - there might be a second hand shop at school run by PTA. Worth checking it out for games kit (even if just for spares) as they get messed up/lost with unfortunate regularity!
Oh & dont forget the gum shield - they are not allowed to play without one.