He'll do great. The competition classes are by height, not breed. However, he has to stay focused on you when off-leash -- he can't run off and sniff in a corner. If his recall isn't already great, practice that.

I have a very active lab/cocker spaniel mix who just loves to run, and who's very easy to train. I think he might like agility. Does anyone here do this? Are there certain physiques or personality traits that lend themselves well to agility? Or can my little mutt dog have a blast, and maybe compete every once in awhile?