Yeah, he looks a bit younger than in the manga. The right side of his cheek is a bit too...sharp, I suppose, you might want to soften it, also with the ears, those should be a little rounder. The hair looks fine, but I think the–-would you call it a rhino horn?--sticks out a bit more. The corners of the eyes are a bit too thick, so if you redraw it, you might want to make them slightly thinner...if you get what I mean. The eyes themselves are a bit too long (as in, top to bottom)...but that's okay. The nose seems a bit awkward, but that's probably because the color isn't quite right...I'm going into editor mode again, aren't I?
The picture overall is pretty good. I can't compare it to mine, because I haven't drawn Heiji, just Conan. My comments above are just comparing it to the anime and manga. Maybe I'll try it. Oh, and I understand that you drew this some time ago, so don't fuss over the comments, it's probably better now.
Maybe I should upload my drawing of Conan...I'll think about it later since I've got something else to do. Good luck! Wait...
*notices awesome possible-avatar and runs off*