For crawling, there's a wide time frame for what is considered normal. Many babies typically start to crawl at anywhere from 6 to 10 months, although some never crawl at all. Instead, they go straight to pulling themselves up and cruising around furniture before learning to walk. The fact that your baby can sit up and move himself around by scooting indicates that his motor development is progressing well; he'll probably crawl in the next month or two. However, if your son still isn't showing more signs of crawling or pulling to stand by 10 months, talk to his doctor.
In the meantime, encourage your baby to crawl (as long as you keep it fun and not too much pressure) by placing his favorite toys just out of reach and taking turns getting them. When it's your turn, retrieve the object by crawling on all fours. Or put on some music and show him how to rock back and forth on all fours to the beat, as this kind of rocking is a precursor to crawling.
Originally published in American Baby magazine, November 2005. Updated 2009.