Hi Kanga! Somehow I missed this WIP, perhaps because I lost my account, thank you for helping restoring it by the way.
Nice work in general, especially the detailing! Some points about the anatomy:
-his posture doesn’t fit his bodytype. Such a posture is for a muscular big man with bulky muscles, where his back muscles are in tone. I think he would need to be more straight, as his muscles are not as big.
His lower part leg muscles seem a bit short, as well as I think you should look for architectural form in “Der nackte mensch” as a reference, as it seems you miss the general form sometimes making those details. I wouldn’t make this line between this muscle, as it is very rarely seen, and only in a tensed state.
His shoulder muscle seems too long. Maybe just a bit, and the posture makes it looking longer than it is. His breast muscles lower line is somewhere where the shoulder muscle ends usually. Maybe his breast muscles are a bit too short?
His elbow bone end is too close to the palm, there is a little space between them with ligament.
His shoulder, if he is so skinny, should have his shoulder bone inner end below be more prominent. At the other side of this end, there should be a little concavity. It’s where the other side of the bone outwards, it goes at the same axis. I’d make it more prominent there.
I think you could make his stomach in the upper part also more concave.
At the back muscles, there the scapula goes, its ridge there is always prominent with its triangular inner area. It’s never covered with muscles, maybe with fat, but not muscles.
The shoulder muscle has 3 sections: frontal, side and back section.The side section is the most bulky, and has those prominent segments where it meets the scapula.

I’d really encourage you looking for Bammes books, I know some of them are now in English, and they have what you need I think - the structure and main form practice, as you are good in detailing, but sometimes miss those basic forms. It’s the problem of American approach for anatomy I think, and Bammes is a clear winner in this area.
http://imageshack.us/photo/my-images/267/refkt.jpg/