Hard to tell. I find that most kids will crawl when ready because they want to check out their extended environment. Having enticing toys out of reach adds incentive. But your? Did not list an age. The range to first crawling runs 8-12 months with many transitioning through crawl/pull to stand and cruse in a several week period. It helps us if you list more info.
Get on the floor. Get on the floor and play with your baby on a quilt or mat. Put his favorite toy just out of reach, so he has to work to get to it. Talk and sing to him to encourage him. Some babies will skip crawling if they find another way to get around (such as rolling). If he still isn't getting mobile, check with your doctor to make sure his development is normal.