Attach his door key to his school bag with a long piece of 1cm elastic, that way he won't lose it and some idiot won't try and take it from his bag (there was a "game" at DCs school where they would try to unzip a bag un-noticed
) It was long enough to reach the door with his bag on the door mat.
Emergency money somewhere deep in his bag and your phone number written into his planner. Even though he may know it panic can erase it in the moment.
Two pencil cases, one for every day items - pen, pencil (we did mechanical ones) ruler, glue stick, pencil sharpener (had to have one despite the mechanical pencil) etc the other for coloured pencils, compass, protractor, so just less used items. These never leave their bags at home, we have duplicates at home so they never go without their equipment.
Totally agree with mellicauli I have two sons one has just finished year 11 and the other is about to start year 9. Only 2 items have gone missing, one was a single PE sock which was a nightmare because the school uniform is only delivered twice a month. So another PE kit the next size up and the tie.
We do a daily "check in" so instead of a how was your day they told us what they did in each lesson, so period 1 in history it was how WWI started, period 2 in English we did a poem called X etc. That way you can talk around the subject and maybe widen their understanding of it.
Join clubs, it is a good way to make friends. My two sons went to a school where they knew no-one. Made friends easily.
School will also do stuff to encourage new friendship groups so don't worry about that so much.
Definitely practise the journey before, talk him through what to do if he misses the train, it is cancelled/delayed. Where he can go for help in the station etc. Having solutions helps them.