Eye-rubbing when sleepy is perfectly normal and as far as I know, astigmatism is a medical condition caused by a problem with the eye, and is not self-inflicted. Unless he startes rubbing his eyes when he is not tired, I wouldn't worry, just make sure his hands are clean. Rubbing his eyes before he falls asleep might just be his routine that he has developed, and he will probably grow out of as he learns to put himself to sleep.
Jen