I wouldn't necessarily think a thought about being autistic is a self esteem issue. Unless there's more to it than you say, I'd see it more as a wondering about his own differences which led to the great discussion you had with him. He's lucky to have such an attentive parent.
I agree with mon on this. Remember also that you know a lot more about what autism is (I'm guessing) than your child does - his information has either come solely from one discussion/book read in class or from what he's heard indirectly on tv/radio or from other friends. One day when autism was discussed in my ds' elementary class he came home, and with the assistance of his youngest sister, "diagnosed" my older dd with autism. If you look at the narrow scope of what he understood symptoms of autism to be, she fit that narrow range of symptoms (from a 9 year old's perspective :)). Kids are curious, kids hear things, kids made connections.
What I would definitely follow up on with your dd is what it is that's behind some of his feelings. It sounds like he's worried about his handwriting. I apologize for not being able to remember everyone's story but it sounds like he's already got a dysgraphia diagnosis (?). My dysgraphic ds did have a lot of struggles with self-esteem in early elementary due to the way his handwriting looked and the difficulty he had producing it. I think that it was really important that we not only talked to him about his feelings, but also did everything we could early on to get him typing (and other accommodations), plus making sure there were times during his school day that he was able to take advantage of his areas of strength. We actually didn't do anywhere near as good of a job at all of that as I wish we could have, but every little thing helps.
Hang in there,
polarbear
