Where do you buy trousers for your teen boy

[Ulezz]

Ds has now grown out of the children’s range and is into men’s sizes but I’m not sure where best to shop.
we tried a pair from m&s but the seam kept ripping on the bottom (he has a round bottom but if I size up the waist it too big)
any ideas for where else we could try?

Primark do a good extra small men's range that fills the gap between childrens and men's. I don't really like buying lots of stuff there as I'm not a fan of the shop, but I've really struggled to find clothes for DS in other men's shops that aren't enormous. I've also got stuff on Next online, and Gap used to do an online teens range, but I don't know if they still do.