My DS (bit older than yours nearly 19 ) likes H&M
Got him some really lovely soft TShirts and some jeans which actually fit him (he's a 26" waist and a 32" leg ). Usually I buy him Tesco jeans but they've had nothing instore and of course now they're not online .
I always have to buy him a 28" waist because that's the smallest so of course he does the teenage boy baggy arsed jeans look ( sigh)
Got him a Genuine Fake North Face jacket in Turkey
Jackets from Costco (no branding but weatherproof with zipped pockets)
Hoodies and sweatshirts from Primark
Nike trainers.
( I went into Sports Direct and asked the salesman "What are 18yo men wearing, I have no clue"
He pointed at his shoes and said "Well I'm 18, I wear Nikes" so DS was "Yeah that'll do me"
TKMaxx do branded underwear but I gave up trying to find ones that were the right size and not garish , so went to H&M.
I do ask him what his mates wear , he just shrugs.
Whereas my DD will tell me exactly what her mates are buying and what she thinks I should buy her ( usually labels )