[@Nallore]
To put it simply he can enter the minds of animals (in this case he only has experience with his fox) and control them. When this occurs, his mind takes over the animal's leaving his body vulnerable. 

I'm thinking that out of everyone his twin sister would know of this secret and would not tell anyone else because there's the chance he could get killed

Edit: he can command without a full take over which won't leave him vulnerable.
Also explains his knack with training animals