I have two mini fillies but am not particularly knowledgable! Hope your little dude is ok, he certainly trumps my girls on the cute scale Feel free to PM me if you ever want to, not sure how much advice I can offer but I can always lend an ear!
These mini's and mini horses are rather prone to poor hind legs, cow hocks or locking patella's, keep him moving as much as possible and don't overfeed him, plenty of roughage and exercise, let him grow naturally.
Thank you,so its best to lead him out ect as opposed to stabling? Unfortunately due to this cr@p weather hes limited and not on grass, he does have a hard standing pen during the day and a roomy stable with a good bed at night. He's on hay (eats about half a slab a day) and a small handful of young stock mix as advised.
As others have said it's best to keep him moving around if stabled. Maybe put his hay in two piles and water bucket separate to encourage movement, walks out in hand are always good and ours seem to enjoy being out and about. He is just so cute!!