am not sherrard but i would guess that it's to "force" him to verbalise. If he can make himself understood with signs, there is a possibility he might not try as hard with speaking. People who sign to communicate rely on other people being able to understand those signs - in the world of work and life, there's more people that understand speech than understand signs.
Sherrard is his parent, and ultimately decides what is best for him. I am sure if it becomes apparent that signs are the way to gobecause the speech isn't improving, she will consider her position then