From my perspective as an ex-boarder - we only wore home clothes on weekends, if weekly a pair of jeans and a t-shirt/jumper is probably enough. Send more than you need at the beginning and then get your child to pack after the first term - they will know better than you what they need.
Don't buy the cheapest canvas hockey boots, not only are they ugly but your child will have wet feet all through Michaelmas.
We had no option but to change our beds on certain days, everyone complied as there was no real option not to. If a boy, everything went to the school laundry, if a girl we had washing machines so could do some of our own (this may be the case for both sexes now). Anything going to school laundry may not come back or not recognisably.
Anything precious should not be taken.
We did not have separated showers, they were all communal (again, this may have changed). Breakfast is possibly the only reasonable meal so stock up - the luminous jam, however revolting looking is really tasty, tea from a massive urn is the best tea ever, the bacon and the sausages are also good. The mashed potato is revolting, as are the scrambled eggs. There will be a grey unidentified meat day - best stock up at the salad bar on those days.
An apple is not a lunch.
Board and card games are a life saver (we were not allowed mobile phones and television was limited) as are really random tasks like rag rug making. Being able to be on your own in a room full of people is a life skill. Playing a musical instrument was a sure fire way to get free time out of the boarding house.
Pack headphones.
I personally didn't get homesick and school holidays were a pain as I much preferred being at school. My family wasn't awful, they were fantastic but going home meant home to their rules, going back to home routines and strangely there's a lot more autonomy and freedom at boarding school.