You're going to have to help me understand how being deaf == having crap grammar IN WRITING.
It's like if you knew American English grammar, and you try to move to use Japanese words with American English grammar, it will not turn out well. He is trying to go from sign language to normal American English, which could have been something he didn't pick up, or does not find value in, let alone trying to transfer a visual language to a written language sharply tied to an audible one. We can't confirm that he had any mental or social disabilities, though, so those options are not really on the table.
It's sad to see him go, as he was generally nice to everyone. He was clingy at times, though, and sought a lot of attention. Also sometimes passive-aggressive, or not so passive. Regardless I do enjoy reading his posts, even if I had to battle through it to find its meaning—he always had something constructive to say.