Not all babies crawl, some just move on to the next stage. If he's strong enough, then stand him against your couch, put a toy just out of reach (if he's right handed, put it to his right) and gently move his feet to the right until he can reach it. Continue to do this until he clues in.
For crawling (which doesn't matter if he crawls or not) put a toy infront of him and brace his legs against the side of your legs until his knees are bent a little. He'll have to "kick off" with his feet to get to the toy/cookie etc. Again, keep doing this until he clues in.
As long as his core is strong and you know which (if any) side is his strongest/weakest he should be able to do this. To gain strenth get down on the floor with him on his belly (works great on the table too, if it hurts your back), get close to him and make him look up at toys.
Hope this helps. Remember all babies are different. I've had babies not crawl, but walk instead.

I babysat for a girl who used to roll like a log to everything she wanted she was a nut, but she eventually learned how to crawl and now she's running around.
With my baby cousin we kind of propped her up on all fours and moved her limbs like she would if she were crawling? I don't know how much it helped but she figured it out pretty quickly

My cousin wouldn't crawl. She would bum shuffle to everything she wanted, she couldn't get her legs to match her arms, it was so cute to watch her though. She eventually started just pulling herself up and and slowly moving to feet and then she walked.

My other cousin would NOT crawl at all, he would cry each time you put him on all fours or propped his arms up. He would not do it. He's now 13months and walking around everywhere!!

I found a couple of articles about why crawling is improtant if anyone is interested:

http://www.myomancy.com/2006/08/how_important_i

http://icpa4kids.org/Wellness-Articles/baby-crawling-how-important-it-really-is.html

I don't think it hurts a baby at all if they don't learn to crawl but it is an important skill that should be encouraged.