I've taken him down a couple dozen times at this point. I've seen the hat twice (didn't get it either time), I've never seen the pet drop, and I don't think anyone on my server has gotten the mount. Some people in the forums are a bit upset about the drop rates on the pet and the hat, given that they're required for the achievement - they don't like the fact that the meta-achievement requires a chance-based event.
I have some good news for those people, via Wow_Ladies: it seems that the Sinister Squashling and the Hallowed Helm both have a chance to come out of the Treat Bags that you can get from trick-or-treating once an hour. This doesn't make it less chance-based, but it does raise your chances, although I imagine the chance to get either of those items from trick-or-treating is very low. Still, if you're interested in getting these items, stay on top of those one-hour cooldowns.