Hiii, nice figure sketch! He doesn't seem too skinny to me, but the legs seem a bit flat, as if they're not very muscular. Muscles bulk out on guys' thighs compared to the pelvis. At the same time, his upper body looks muscular, so this might make the legs seem skinny by comparison. Here's a picture to illustrate what I mean, compare the shape of the legs to the pelvis and chest:
(English is not my first language, so hopefully this makes sense :))
Awesome stuff so far!