This is a great book, Usborne Going to Hospital The boy, Sam, in it has his ears fixed 
This is the video thats our hospital made about going for surgery - I watched it with my older 2 before their ops (not relevant for the 2.5 yr old)
Contact your hospital and find out if they have a play specialist, the are there to help you prepare your ds (plus the more they help the better it is for them!) You also need lots of information, the calmer and more prepared you are the more relaxed your ds will be.
I'm not sure there is a 'nasty' side to the op, I've always been honest with mine but used language they can understand, they have a 'magic' sleep and get their ears fixed, it will hurt a bit but it'll be worth it so they can hear etc.
Also is he unsettled because of his hearing or knowing about the operation? ds2, particularly, was clingy, tearful and tired before his op but a different child after and it was down to how disoreintating he found his impaired hearing. I've used nursery/School to help prepare all 3 of mine, they may also have resources/books/dvd's you can borrow even if you don't want them to discuss it with him, its also worth letting them know how much you want ds to know so they don't tell him something they shouldn't or you feel uncomfortable with.
Finally find out if there will be toys around on the day and what food is there afterwards, I always take a huge bag of stuff, there is always a lot of hanging around so you need distractions. (don't forget to take something for you too!) I