Not crawling

But has he started something else? For example, sitting up, or pulling himself up, flipping, standing, Eating? etc. if so, then you don't have to worry, some babies just skip crawling or sitting etc altogether and proceed to the next step. if he has not, and had not yet flipped, not sat, nor mumble any words, nor has a lot of things he had not yet done, then perhaps you should look for a PD to evaluate.