I don't see how, since you can learn grammar from text. It'd be hard to teach letters and spelling sure, but grammar can be taught using text.
Grammar varies by whatever language you use. For example, the usage of adjectives may vary. In some languages, they go before the noun, in others they go after. In some languages you put the two words together to make one.
You can teach English grammar using text, yes, but perhaps it's difficult seeing as he has only used Sign Language. Trying to type in proper English is probably his equivalent to learning a second language. Maybe he's reading our text like we are reading his, just putting familiar terms together and trying to use common sense to fill in the blanks.
Also, Furling, I think this is a wonderful idea. :)