Achondroplasic, but not badly affected. It is (or was before the American invasion!
) the commonest form of dwarfism in the UK but I would not say it is the "mildest" form- maybe because I have seen a lot of severe Achondroplasics and not that many Brachiocephalics (yet) I feel this way but I think this boy, complete with his "ballet position " front and rear has a pretty good life ahead. I would consult with a remedial farrier, but I do not think there will be much, if anything, you can do about his legs as they are a basic conformational fault, born with them and I would think has to live with the, The Magic Shoes work wonders if the hoof turns over or under, but I am not sure they will help in this case as his feet look to be in pretty good shape...
Welcome to the Forum, where the only criteria is that the horse is loved, not what it looks like!