This has got to be the best Superboy Cosplay I've seen so far. All the other ones were chicks.... anyway, you did great. You look like you honestly just stepped off a scene from a live action version of YJ.
The joke is that I was really fat when I was a teenager, so all I could do to lose weight was working out like hell. And now I can never stop that because everytime I stop training for like one month I gain 3-4kg of fat again (also I love food lol)
So having a decent look as superboy is just my dont-get-fat-again-workout
I know a lot of people that are slim having problems in gaining mass. If you really want to try it, go to gym like 3-4 times per week and don't forget eating loads of protein-stuff and maybe carbos before you start.
But of course it's always a really hard piece of work, I'm training for about 7 years now.