Just love on the little guy all you can, he'll be feeling better in a few days.
My Sully had the same thing. At about a year and a half I got him fixed. The vet phoned me that day after I had dropped him off and said one of his nads hadn't descended and that it would be $20 extra.... as if I wouldn't get it done because of money. Sheesh. Anyway, he felt like total shit after, I just wrapped hiim in a blanket, put him in the car and took him home. Poor little guy (he was still fairly little then) even peed blood for a day or two. But he came around in about 3 days and was back to his cheery self, mostly.
The procedure did calm his hyperness a little, which was a good thing.
Here he is as of last year.....
<img SRC="http://members.shaw.ca/billy1-99/pics/sully_2002_03.jpg">