DS already does a lot of bike packing and wild camping. It is not a list for everyone - he knows how little he can get away with while still remaining safe & in good humour. He is comfortable not washing for days on end and going a little hungry until he gets to the next open shop. It will be different for everyone and does not come cheap.
For food - Fire Pot dehydrated meals, M&Ms, haribo, pork pies / sausage rolls, a couple of the hot chocolate sachets & extra sugar, Some cake that can withstand getting squished. He aims to get as many calories in for the least amount of weight and a balance between sweet & savoury. He has tried gels but they don't agree with him. Depending on what he is going he aims for 3,500 - 8,000 cal per day. DofE he is going to go nearer 3,500.
Sleeping - cut down carry mat (long enough to reach between shoulders & hips), Lightweight hydrophobic down sleeping bag, Lightweight single skin 1 person tent. If it was allowed he would just take a bivvy bag.
Clothes - full waterproofs, gillet, pertex fibre fill puffer jacket, hat, buff, gloves, waterproof over mits, sealskin socks, normal socks, walking trousers, T shirt, marino base layers (long sleeve top & tights), fell shoes (mud claws most likely), boxer shorts. He will ware all but the marino base layers during the day. At night he will take off the wet things (ideally the waterproofs) and sleep in as much as he needs to to keep warm(ish) - the hat or buff is essential in this!
Other stuff - lighter, head torch, flint, spork, half a toothbrush, toothpaste, whistle, map, compass, GPS, spare batteries and mobile, small pack of tissues, water bottle and depending where the water purifier. And some money.... for when he gets to the end or if it goes tits up & they end up off the hill in the wrong place.
All in a 35l rucksack or smaller depending on how small he can pack it.