Welcome to the boards 00x
I like your character. He looks like a wrestler to me.
As mentioned, he is pillowshaded, but he also feels a bit cluttered. I believe it's because you've tried to convey everything with lines.
It's better to use volumes, especially on sprites this size since there's not much room for actual lines.
As for drawing heads (and anatomy in general) I'd suggest to get a hold of some books by Andrew Loomis. It's very good stuff and will help you considerably.
Looking at what you're currently doing I'd suggest the book "Fun with a Pencil"
I also did an edit of your sprite. I tried changing his legs posing a little as well, to give him a more wrestler-y appearance.
I also flattened his feet.
Since he's so small it looked like he didn't really stand on anything.EDIT// I meant, since the character is so small it's okay to make the feet flat. Makes i look more like they are actually standing on something.