I started pulling my eyebrows out in my sleep last year due to extreme stress and anxiety. It's actually quite embarrassing because I have no control over it, it's painful and it wakes me up when I do it, plus I looked strange with half an eyebrow missing. I have had to resort to putting tape over my eyebrows at night when it flares up.
Have a read of this:
Wearing mittens to bed or a soft hat to bed may help a bit, but he would probably pull them off in his sleep. In my experience, they would probably need to be taped down somehow (which I personally would not do to a child).
ETA the Wiki article says that in small children the condition is usually self correcting, and they will grow out of it.