Rugby shorts are short, less to grab, have a pocket for gum shield, but everyone tucks it in their socks, footy shorts are longer and can have zipped pockets, rugby can't because it can scratch.
Rugby shorts are heavy duty cotton or polyester as they get tugged around and footy shorts are light weight.
Just buy a pair of ok quality black sports shorts if he isn't likely to be on the teams he won't need special shorts.
You will need the rugby shirt though, it's used for footy and hockey too in winter and the pe top I'm afraid as it s crested.
By the end of the academic year they relax about non marking trainers as the hall has a good polish over summer, so if he goes up a size over spring/summer don't panic as cricket,rounders,athletics all outside in trainers.
I will await your questions on winter skins.