As long as he didn't eat the pit he will probably be ok. There's been a lot of debate on the whole avocado thing, and while there is a compound that is toxic to some small animals it actually appears to not be bad for occasional consumption by dogs. Here's a post by Dr. Jean Dodds, a vet who specializes in toxic substances and how to minimize their effects. She actually does a lot to help reduce liver toxicity from epilepsy meds.
Canine Hydrocephalus Support on Facebook
"Being the parent of a special-needs pet means living your life constantly poised on the edge of a double-edged sword. On the one hand, you become a fierce defender of the ways in which your little one is perfectly ordinary — all the things he or she can do that are just like what everybody else does. And yet, you never lose sight of how absolutely extraordinary that very ordinariness is, how difficult, remarkable and rewarding that fight to be 'just like everybody else' has been." -Gwen Cooper, "Homer's Odyssey"
Shadow - 03/01/2013 - 10/02/2014