Thats quite awesome! Personally, I'd say when he's holding the boulder higher than shoulder height I'd make the arms shudder a bit or buckle a little under its strain. I guess also the "throwing-arc" of the boulder seems a little too direct for something of that weight, as I imagine it'd pretty much only clear him a little.
Bear in mind though, I am no animator, so I'm probably wrong.
None the less, brilliant work. I particularly like how you got his tail to reflect his various stages of lifting.