Trimming should be done every two weeks to keep the feet turned in. But I must caution you, trimming may turn the hoof in, but not the stifle. If the stifles grow normally after the horse muscles up and gets stronger, the horse will toe in. It could be a birth defect or due to overextension of the hind limbs. The round ligament holds this joint together.
Prognosis is guarded at best.
Then again, the foal just may grow out of it. A few of his brothers had the same symptoms and grew out of it, but will still rest with the fetlock broke over. He has a steep pastern. He had a lot of heel when we I got him. Some people tell me to lower the heels and some say to keep him as steep as we can.