No need to be down on yourself here, this looks fantastic! The strength of the lighting and the intensity of the face look great to me.
In regard to the muscles, I think either a thicker neck and wider shoulders to fit his larger arms or thinner forearms to fit his smaller frame would clear up any issues. Love the colors and the overall rendering style though.
I think you did great. We are our own worst critic. Well at least you should be if one ever wishes to be better.