If they are having packed lunches make sure if you put the food into tubs that your DS can open them, peel oranges, pop the lid on drinks bottles (Some are really stiff) etc. If on school dinners can use knife and fork.
Go to the toilet entirely independently.
Practice changing into PE kit - and putting their school clothes/shoes safely into their PE bag (so they don't get lost amongst 30 other thrown off uniforms).
Putting on (and preferably doing up) own coat.
Recognising name can be useful (e.g. on peg, book tray etc)
I think grants has got it spot on. Work on the practical life skills, borrow some books from the library about starting school (there's a lovely Shirley Hughes one) so you can start exploring together what it might be like. Don't worry about the rest, he'll have a lovely time, especially if he knows that his mum is happy and confident about him starting.
Don't go on about it too much. Make sure he can do up his buttons, wipe his own bottom, get dressed and undressed by himself. That's it really, they need to just relax and enjoy themselves into it, don't expect too much, give the teachers chance to get to know him and the class.