Yes, looks male, and he does need to fill out some. A flat or even slightly concave belly is one thing, but as best as I can describe it is if you compare with Duane's photos, while the belly might be just slightly concave, the underside of the fish lines up smoothly with the lower jaw. Not the case with the green terror, his jaw is noticeably deeper than his body. So, no, you don't need or want a round belly but he does need some conditioning.
Hard to say about the fins without seeing them extended, but while he's small it almost looks like how the fins on some older males can curl up a bit. Hard to say if that's just something with him as an individual fish or if he's stunted, and older than his size implies.