I remember at that age wanting a completely impractical bag and being allowed to get it with my own money but having it made clear a further bag wouldn't be bought for me for the rest of the year.
Boy did I hate that bag all year even though I was far too proud to ever admit it.
Sometimes the best lessons in life come from being allowed to make mistakes that won't really hurt us in the long run.
Could you explain your concerns to DS and try to make a plan with him to help him learn how to make good decisions but ultimately allow him to fail if its really important to him given he has his old rucksack as a back up?
Not that my blasted school bag was the last impractical bag I ever bought in my life!!!!
I have the same problem. I make him take a backpack on days when he has PE, as he has lost so many PE kits - if it's all in one bag its less easy to lose. I don't understand how he can want to carry everything in a messenger bag - it's so heavy when he's got all his stuff ( the dictionary, reading book, pencil case, exercise books, language dictionary etc) but apparently a backpack is really not cool. Like Tantrums says, you have to pick your battles.
No blazers here (can barely get him to wear a coat in winter) so it all has to be carried in a bag. Yes PE kit can go in separate bag it just seems such a pita! I'm so not into messenger bags, especially if he ends up having to carry everything separately anyway. In France (where we used to live) secondary kids had so much stuff to carry it was actually de rigeur to have bags on wheels . Can't see that going down too well .
DS 17 has this osprey bag It's very roomy and also has extra support by way of a waist strap if you want to use it. Probably not that fashionable but DS is very pleased with it, after obsessing about it for ages I do spend an inordinate of time with him in shops like Go and Cotswolds looking at bags as he is a bit of an outdoor adventure nut.
Luckily the boys at DS1 (and soon DS2)'s school all seem to carry backpacks, even in Y11 and sixth form. They mostly wear them on both shoulders too. The messenger bags are becoming more widespread at primary school, especially in Y5 & Y6, but then they have a lot less to carry at that age.
My DS got an adidas messenger bag in year 7 and it was fine. He got his lunch and books in, no problem. He did carry his PE kit seperately but as he had a locker, he hardly ever brought it home anyway (very smelly ). He has asked for a new one as his got a bit scuffed around the edges from him treating it as a football, but he could probably manage another year.
Surely the solution for someone going into y10 is to give them the budget you can afford/think is reasonable and leave them to it!? He's asked to buy a new bag because he's realised the current one is uncool - surely that's likely to be the case with the new one if its picked out by his mum!