I made some edits to his posture, added some folds to the clothing (particularly around the knees), gave the buttons some highlights, and darkened a few shadows.

What scale are you working at? I feel like this would make more sense at a smaller scale than what you've got going on here - there's a lot of wasted pixel space going on right now, which gives it an MS Paint kind of feel instead of a more polished look.

EDIT: Maybe something like this?

I feel like you picked each colour in a vacuum instead of relating it to other colours in the image. If you want a new color, instead of thinking "Oh, I need a green. Let's pick this green, sure why not?" take an existing colour you already have and hue/saturation shift it to become a green that matches it to an extent.

I did some colour swapping to make the image more harmonious. I've always liked this palette tutorial, which illustrates the concept of going from one colour to another.

I snagged the original image from the link to the html site, so apologies for the large scale. It didn't seem to play nice when I was resizing it in photochop.

The light source seems pretty inconsistent. I think it would be helpful for you to pick a specific spot, probably upper right corner would work best here, and then shade according to that.

Additionally, the AA on the outside seems a little pointless here. It just makes it look a bit messy, especially since it's so much darker.

For the anatomy, consider putting the shoulder a bit further back from the head? Or having the mouth be a bit further out.

I really like the front hands, though. I love how you've really got the gorilla knuckle-stance down. A little more contrast might be good if it's meant to be seen at 1x, but it looks fantastic at 2x zoom.