He's sleeping on his back. The Back to Sleep campaign, launched in 1994, has done a world of good in preventing SIDS. But a side effect has been the older age at which babies today crawl. And some babies never do crawl. The good news is that it doesn't matter. Your baby does not have to crawl in order to learn to walk.
He's not getting enough tummy time. Since your baby's spending so much time on his back, try to give him some incentive to crawl. Here's where tummy time comes in. Scatter his favorite toys around him on the floor just out of his reach. Sure, he may balk at first, but learning to wiggle on his tummy is one step closer to crawling.
He's found other ways to move around. Observe how your baby gets around the house during the day. Does he scoot along on his bottom, roll across the floor, or cruise while holding onto the furniture? If so, then he's moving in the right direction. He could also wake up one morning and just start walking. But if he hasn't found any way of getting around by his first birthday, talk to your pediatrician.