Kids fall - their bodies are designed to take it.
Is he hurt when he cries, or is he just startled? DS will topple over sometimes when he's sitting or crawling, and the first couple times he cried - I scooped him up, gave him a kiss and told him he was fine (while checking for a bump) and plopped him back down. If I didn't make a big deal out of it, he stopped doing so as well. Now when it happens, he gets up and keeps going on his own. There was one time since then that he really did whack his head pretty hard and started wailing, but since he doesn't cry at the small bumps anymore I knew he was really in pain.