Don't fix this. He is too old to fix.. and this is a common issue of growth in foals. If you want a horse to grow straight.. they need to be trimmed as foals (foals spread their legs to reach the ground and wear the inside of the hoof).
Without regular trimming, most foals grow into horses that will toe out a bit.
You can help him by keeping his feet trimmed and putting plates (light shoes) on the front so he cannot wear the inside down. This will support his hoof walls as well. It won't make him straight.. and that is OK. Making him straight with corrective trimming at this stage will put stress on his joints and would likely lame him.. if not now, in the future..
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer