Please don’t stress, these things happen.
They have water in school so they won’t go thirsty if they’ve forgotten their bottle.
Also don’t worry if you can’t get shorts. I’ve spent so much time worrying about whether they’ll be too hot or too cold and they’re just fine. Your son will be ok in trousers.
Agree about the Charlie idea.
We’ve forgotten water bottles, broken shoes in the morning, once got dd dressed in uniform on Pe day and realised last minute, forgot a non uniform and had to take clothes for dd to change into. I’ve had to drop off coats, extra shoes too. These things will happen.
We leave our water bottle out each night, we do the mental checklist in the morning before leaving and the kids also know the drill, important dates (non uniform, inset days, etc) go in my calendar and phone calendar as there are lots of them, uniform is ironed and hung up each week in a separate cupboard and I have a separate school drawer for tights, socks, Pe trousers and shorts so things can be grabbed each night. I avoid packed lunch as much as possible to make life easier 
I’d try to get up a bit earlier if you can so it gives you time to deal with mishaps…unfortunately I start out with good intentions but end up being the parent running into school 5mins after everyone’s in 