Was he breach? My neice's daughter was breech. Her ped told her that most breech babies never crawl, they just sort of bounce on their butt. They don't know why this is, but it is for some reason.
If not, try more tummy time with toys that he likes. It may help him develop those muscles. Also, keep in mind, some kiddos have their own plan (they don't read the manuals). My husband's nieces didn't crawl until closer to a year (or after). Their peds were not concerned. Milestones are on a bell curve, meaning there's a range of normal behavior.