My girl didn't even look like she was ever growing to crawl at nine months, then at 10 and a half months she started commando crawling and a week later the real deal. They'll do it when they're good and ready and believe me it's easier when they can just roll. Now I have a permanant follower around the house.
A friends little girl is nearly 18 months and still doesn't crawl or walk. They had her tested to make sure nothing was wrong, but it was all clear. She's just lazy, why crawl and use all that energy, when they can just roll everywhere.
It'll happen when he's ready and then you'll wonder why you wanted them to, as he'll get into everything and discover things in your house you didn't think of.
Lisa, SA, baby girl (4.5.03)