Auditory learning is the weakest type of learning.
You'd learn sign language through visual (sight) /kinesthetic (touch) methods and then apply the signs to written words.
Boom, writing.
Further more, after being exposed to grammar as much as he have has, (5 year olds can't internet) wouldn't he have slowly picked it up?
AKA monkey see, monkey do.
edit: Sign Language grammar appears to be Yoda-speak. (no offense)
A troll, you must not be, you mustn't.