My middle one (16) has significant disorganisation issues. No SEN.
We have spent years working on this with him. It has improved but he could still lose something in an otherwise empty room.
One of his issues has been he is quite timid so wouldn't chase things up for himself. But he learnt that this was necessary.
Lost labeled uniform was a big issue for us too. We lost so much I spoke with the teacher and school admin and agreed he could borrow from unclaimed lost property when things went missing.
Over time he has become better at locating lost items. He is a loveable affectionate young man but naturally lacks organisation - he goes too much with the laid back flow!
I think we all need to take personal responsibility and whilst loosing 9.5 weeks pocket money is a major punishment, I wonder if you could put a hold on spending that money for 9.5 weeks (or a certain period of time) and if he endevours to chase up the lost item x times he earns back the credit.
If he could engage in making a list of all the ways he could help to chase up/ look for/ locate lost items.
In parallel he could have a laminated check list on his book bag for stuff he needs to check he has when he leaves school each day. At first who ever collects would probably need to stand and run through the list with him - did you have a coat today, did you have a jumper, was it PE day, are your trainers back in your bag, was it forest school does that kit need to come home.
Don't do it for him or check for him, initiate him reading and running through the list until one day he comes out and informs you he has his coat, jumper, PE kit because he's picked up that he needs to do this EVERY DAY.
Lets face it, he will keep losing things and learning basic organisation skills isn't coming naturally so he needs to be taught to find a way to do it for himself. Without help, in the blink of an eye, he'll be a disorganised teen grunting at you when you're at the end of your teather with a uniform infringement letter from school.