Ok so for this one I'm looking for advice on how to help this horse fill out and muscle better. In the past my family has only had QH, TWH, and Spotted Saddle horses. We never had anything that took much work to muscle, and we would love to help this guy look as awesome as I know he could. This horse came to us underweight (first picture) he has gained weight, but I'm not sure how to help him fill the muscles in his neck out.
Give me all the advice you can :) He is my little brother's horse, but my mother and step-father are both working with him to do whatever they can to help this big guy reach his potential. He mostly neckreins, but works off leg cues better than anything.
He will not take the right lead, he always takes the left, (and is really smooth doing it) Advice on that would be really good too. I've never had one that wouldn't take it up even when pushed in a circle and given the cues. We tried letting him learn the cue for the right lead and hoping that would help carry over, but he still won't take his right lead after almost 6 months of work.