Hi S.,
When I read your post I thought you were talking about my son and most of his friends.
This is very much a developmental issue for many boys. Not only does my son forget his clothes, it's his lunch box, homework, backpack etc... when he was 12 and younger.
He is now a freshman in high school, (14) and still forgets and loses things. But we try to remind him all the time, to check his stuff. The funny thing is that he has found kind friends who pick after him and remind him on what he has forgotten or lost.(mostly girls)
Yes, I agree with other parents to have him go jacket-less or pay for one with his own money. But if he's anything like my son, he gets angry with himself when he loses or forgets things. Then does it again. Doctor says it's puberty, and some boys brain's regress back to a three year old...
Don't know if that is true, but it does get better as they get older.
I'm in contact with many high school and college students and I constantly ask the boys what they did when they were your son's age. And majority give me stories exactly like yours, how their parents would get so frustrated with them, and that they didn't do it on purpose - they just forgot.
I know this doesn't give you a solution but maybe this may make you feel that you are not alone.
Good luck, love him for what he is...