Ds has had grommets 3 times now. They've always put the canular in his hand after they've knocked him out with the gas. So warn him he make wake up with something in his hand he mustn't touch.
Biggest thing for him is do not promise you will be in the room when he wakes. They said I could be the first time, (he was 20 months) but he woke too quickly. Since then they have changed so they only let you in when they've come round and are okay. In his case it takes some time to get to that state. So he wakes in a strange room with no one he knows.
The second time he leapt into the arms of the nearest nurse, hugged her, then said "you're not mummy" and kicked her. 
Luckily she thought this quite funny.
I found when he had grommets at 3.6yo it was actually the hardest. Because he was old enough to understand a bit and worry, but too young to be able to cope with the worry or understand why it's happening.
So we had:
- He walked into hospital and anounced "Mummy is trying to starve me to death, she won't give me any food" To which the nurse said "good" and he was cross.
- When the anathetist came to talk to him he refused to do anything. When he suggested a game of blowing the balloon up (get him to breathe the gas) he was suspicious enough to refuse and not old enough to see it as a fun game that he could.
- Holding down a strong 3.6yo to put the gas mask on was not good.
- He woke up very quickly and didn't want to stay in bed until they felt he was well enough.
- He was most put out and interested in the "funny thing" in his hand and nearly pulled it out several times. Had to keep stopping him pulling at it.
There is a Mr Tumble in hospital episode if you can find it which is quite good. And the Osbourne first experiences "Going to Hospital" is about a boy going to hospital for a sore ear.
Also don't say "they're making your ear better". Because he'll go to sleep with his ear feeling fine and wake up in pain. Not huge pain, but enough to feel that they've made it worse not better.
The hospital always provides stuff for when he wakes. Don't let him eat too much at first and then be sick. And if he's there for dinner go fairly quickly otherwise you tend to find all the easy to eat for little ones meals are gone and you're left with something that sounds fantastically grand but no children want when they're ill.
Our hospital has a play therepist to talk them through what will happen. Ds refused to cooperate at 3.6yo (new toys in the play room much more interesting) but it was really good when he went in the next time.