The body anatomy is looking good, so far. Suggestions...
Probably it is intended by you, but the head seems too small in relation to the rest of the body.
about the head
You're actually quite correct about the head being very small. I wanted to play with an odd proportion set I ran across in an old (read 1870s) art book from Dover Publishing where they had a proportion set they referred to as "olympian" where the total height of the body was 10-11 head lengths and I wanted to see what that does to the silhouette and "feel" of the sculpt. One thing I'm kicking around is stealing a proportion idea from the Michelangelo "David", where the sculptor gave the hands a very large proportion relative to body size to suggest strength (I wish I could claim this was a personal observation but John Brown over at Gnomon was the one who pointed it to out to us in his sculpture class). I think that by possibly increasing the hand size and perhaps the arm length slightly it may make for an interesting silhouette, but I will also try your suggestion of increasing size of head to body. The big thing I'm trying to stay away from is too much of a simian body proportion (i.e.5 heads to body or the like) but I think you may have a valid point in that I pushed too far the other way, so it's off to try your idea out. Thanks for the help!
Hey nice work so far. Im new to zbrush but heres my 2 cents. I would suggest making his pectorals bigger looks a bit strange especialy from side view that his ribcage sticks out further. Also adding in the ribs to follow the rib cage would improve the model maybe even pull in the waist a bit more. I would also suggest thickening up his lower jaw doesnt look like there is any room for a jaw bone in there at the moment. I think the more you exagerate an alien model like this the better but still think about the anatomy of it to make it believable.