Never done Reading but have done plenty of festivals. How is he usually in regards to drink and drugs? I've smoked/smoke weed (outside of festivals) but never had any inclination to even try other stuff, so if he's sensible and has his head screwed on he will most likely be ok. He'll probably have a mad first night, be sick etc.
Do people take rucksacks? Wheelie cases?
Wheelie cases are a PITA to pull across mud. Just a rucksack (a proper hiking one can fit LOADS in, with the straps around waist etc) will do. Clothes in carrier bags incase it rains and the bag gets wet! Has saved me a few times. Bundle up outfits together for quick changes.
Phone and wallet at bottom of sleeping bag when sleeping
Consider swapping for cheap phone
DEFINITELY. You don't get a lot of signal anyway so internet ect is usually useless.
He's a guy so can easily use toilets onsite (troughs for peeing, and IME portaloos aren't that bad if you hold your nose (or have hayfever!) and get in and out!) I never see the point going offsite at a festival as local places push up prices anyway and a local cafe doesn't need 6 teenagers clogging the toilet with massive shits! Like most teenagers they probably wont surface before 11am and will have a mars bar for breakfast lol.
Foldable poncho has always been great for me to stave off showers. and you can whip it off when the sun makes an appearance and it doubles up as something to sit on!
Make sure he takes wellies / boots and proper socks! If he starts getting damp feet, socks on, plastic bag over sock and into wellies, will keep feet dry if his shoes aren't properly watertight.
You've had some great advice and he will have a great time. Although he will probably take too much stuff he will never use, and all of your lovely information will go out of the window. He'll come home bedraggled, hungry, hungover, tired, nauseated, skint, probably lost something but he'll want to do it all over again the next year.
get a disposable tent and leave it behind
Please don't encourage him (or anyone) to do this, tents are not disposable. It's fucking revolting and costs hundreds of thousands to tidy up after festivals. Yes some (very small number and only at certain festivals) will have charities that come in to collect good tents but most end up in landfill! Encourage him to be respectful of his surroundings and keep binbags around to put cans and rubbish straight into (probably not going to work with pissed up teenagers but you can try!). I HATE seeing rubbish at festivals it's grotty and so fucking disrespectful. I've started going to festivals that have very green policies and it's brilliant seeing people actually putting rubbish in bins (and recycle) and the campsites being pristine when packing down. /rant