"As for merging with your wolf; your wolf is caged deep within your core, where your soul is. It will take time, but the more you understand the split between your soul, the more you can dismantle it. When that happens, that’s when you will be whole again." I won’t lie, this was mostly speculation. The notes left behind depicted the ritual as binding the wolf within the soul. My guess was that Fenrir found out how to merge with his wolf when he was trying to learn magic. The man never had a chance for a proper education like Remus and was certainly the kind of man to force it if he thought he could. 