Abs appear when you're at about 10% and below, although this'll obviously vary slightly depending on whether or not you're a bodybuilder (bodybuilders have a more efficient method of building ab size which is not actually the approach most core programs use, and why a lot of "core programs" typically don't work.) At this level you're going to find men are actually x-shaped.
http://askthetrainer.com/exercise-information.htmlIgnore the whole "click to see exercises" garbage, as this website may or may not be valid in that department, but look at the anatomy. Now, because you're drawing a character with low body fat, certain characteristics are going to start to stand out, although not all will.
The first is that at that body fat, your traps are definitely going to be visible, they're only really going to be gone for fat people. The traps are those triangular looking "neck muscles" (they're actually bigger than your abs, they extend into your back, but that's not relevant here.) Your character is missing his traps. This actually cuts down on his shoulder size and shortens the neck, which will shorten everything else accordingly and make him look slightly off.
The second, look at the abs on the image. Those ones aren't entirely accurate, there are actually 8, but the inaccuracy is excusable because abs vary from person to person. you'll find several images of bodybuilders with extremely thin abs that end well above their bellybutton. I have much taller abs, that extend well below the bellybutton. So with this, you have a lot of leeway. But look to the sides of the abs. You're going to find a lot of horizontal bands of muscle. You don't need to know anything about these, but look at their shape. You'll find they actually come inwards, almost like a girl's hourglass, and then jut back out. How exaggerated this is will depend on how big your character is, but this isn't the hulk and he doesn't have monster lats, so you'll be fine if it's just a subtle change.
This can be more easily done with that sketch by broadening the pecs, he's really rectangular, but he's thin so you can't afford to do this subtractively. Broadening the chest, even if just a little, will help bring out the X shape while making him look more toned, but not necessarily big.
He doesn't seem like a bodybuilder, and the picture isn't shaded, so your arms are generally fine as is, but if you're going with something more athletic you'll have to make sure the deltoids (shoulders) have some more definition. You can do this by making them larger and extending them further down, or by slimming down his biceps and triceps if you prefer.
http://fireemblemblog.files.wordpress.com/2008/06/volug.jpgTake a look at Volug, for example. His arms mostly, although you can still find he has a good x-shape and nice traps. The top shoulder muscles are the deltoids, they're bigger and stronger than the rest of the arm, and they're going to show up if you're at a low bodyfat. You'll also find the arm curves around the biceps and triceps. This is what gives him that athletic, toned look. It's common for people to have deltoids kind of merge with their biceps and triceps, but that's because they're out of shape. Someone who's in shape is going to have a visible difference.
Sorry if my posts might focus too much on anatomy or seem a bit nitpicky, if you haven't noticed I'm huge into the gym scene so I try to be really aware of muscles and body shape so I can find out what I should look like and where I need to get better, so I tend to notice those normally little things.
As for some general advice, experiment with shading. If you're not comfortable with it yet, it's more than fine to wait, but you'll find they help you add a lot of dimension you can't otherwise get.
Another general tip is to experiment with is to draw your characters naked first and then clothe them (my girlfriend likes to do this. She leaves out the privates, sorry guys) or, what I prefer, is to block in your clothing and then draw their body underneath to see if it's still proportional. The problem with clothes is that since they conceal so much and since they're so simple, they can sometimes warp your characters body.