Elvis,
My son didn't have an undescended testicle but did need surgery for an inguinal hernia when he was 2 and a bit. Just like Rivi's son, he bounced back really quickly from the op - he was crawling around under the bed when the consultant came to see him shortly after he came round!
We prepared him as best we could for the experience without frightening him, and told him there would be a present waiting for him after it was all over (Henry for his Brio train set, something he really wanted) as a reward for being brave. When the day came the nurses were brilliant with him. He took his koala bear (his favourite cuddly) with him. They gave her a wrist band just like his, and she went down to the operating theatre tucked up on the trolley with him.
The worst bit for me was when they knocked him out - seeing him go wobbly and then unconcious made me really shaky, then seeing his incredibly small form being wheeled into theatre - I shall never forget it. I'd brought a book to read while the op was going on but I never managed to get through a page.
When he came round he threw off his bedclothes, then took off his hospital gown and his nappy (not sure why exactly, but it seemed important to him), then asked where his present was. Within a couple of hours of the op he had demanded and downed a large plate of sausage, beans and chips and obviously felt fine. They sent us home with various painkillers (which he never needed) and now, nearly 2 years down the line, you can't even see the scar unless you look very hard - the glue stuff they use now instead of stitches is incredible.
Before the op the consultant told me that the whole thing would bother us far more than it would bother our son, and he was absolutely right. I think that we tried so hard to be totally matter of fact and calm about it all that he considered the whole experience as something totally normal, and even rather enjoyable (the being made a fuss off bit, anyway!).