Looks good thus far. Only a few issues that I noticed, aside from the arms.

First, the pectoral muscles seem a bit flat in the center. I'd add some more detail in there, to get a better, more natural look. Next, the shoulder muscles (deltoids, right?) look a bit big, when compared to the thinness of the rest of the body. I think that could be accurate when the arms are down to his side, but not with the arms out to the side (default pose). They should be pulled down towards the shoulder blades in such a pose. But, I'm no anatomical expert:p .

But, as is, I think the back and torso look great. He's got a Spiderman type look going on, and I think you're off to a great start.