If you are talking the Purina Mini and Pony feed, it's really not suitable for growing youngsters, it doesn't have the right balance for "babies". If you've already opened it, it'll be ok, just not quite nutritious enough for balanced growth; if you haven't opened it, return it and get something else. [What do you feed your full size horses, perhaps something you already feed will work in mini size portion?]
A good farrier will go a long ways in helping his legs by keeping his hooves balanced. Many young horses are cow hocked, especially if thin, as they don't have the muscle mass yet to help straight out; good balanced hoof trims, balanced diet and basic exercise will help (no lunging, circles are too hard on young growing joints), cavalettis are a good choice (straight line walk/trot).
He's thin, so no clipping until the weather is much better and/or he has gained some weight. Go slow with the weight gain, good grass hay and a little alfalfa will go a long way, and we'll figure out a good supplement/feed to go with the forage that will be appropriate for his age.