my 2 yr old dd recently had day surgery, the nil by mouth was ok (until midday, when all hell broke loose, but unlikely you'll wait that long). I kept her out of sight of any food / drink on the day.
There should be a play room with toys, but probably not so much stuff for his age, so could take a few favourite toys to entertain him.
Obviously take nappies (sorry if that's too obvious, the nurse looked visibly relieved when we said we had them, so some people must not bring them).
Afterwards they'll only let you home if he's drunk some milk and done a wee.
I was glad of taking some food and drink for myself, as couldn't eat breakfast before we left. Once we were on the ward, I didn't want to go to the cafe in case we were called for surgery.
And also was very glad of dh being there, so if you can get someone to come with you, that would be ideal.
TBH if you're seen early, then it should be fine - dd was quite happy watching all the bustle of the ward.
Hope it all goes well, I know how stressful it is. I'll post more if I think of anything.