Put the avocado in a greenhouse and keep Biggle indoors? Or I have some chilli pepper plants you could try using to teach him not to nibble growing things.
I found a split mango stone with a little bit of a shoot in our compost bin, but it didn't grow any further when I put it in a pot, so it's back in the other bin (which is nearly full - I must finish sieving the first one).
Steph Bell had an avocado growing in a pot on her mantlepiece at college.