hey Ixis
have in mind that if your gonna make a sprite that big it will be double work to make it walk and any animations you want to make.
Anyways, The head looks like he is really fat. Push up his cheeks, make his year bigger; the top part of the year is aligned with the center of the eyes and its bottom with the mouth.
The hair seems a bit polygonal, but it's ok
The underwear doesn't fit at all with his position, check that out again. If you watch the drawing carefully you'll see the volume of his belly and his legs as well. They make a curved line that must dominate his pelvis, but not his genitals. By this I'm telling you that the top line of his underwear must be curved.
Make his neck a bit thinner as well as his arms and legs.
Don't make his legs bent, make them strait as the drawing and check his hands and foot.
I assume that you'll add colors to this piece.
Apart that its cool and it has future
cheers!