I have 3 kids, my oldest who is 13 didn't crawl AT ALL. He skipped it all together. He had literally no interest in it. He would actually get kinda mad if you had him on his tummy for too long. He NEVER crawled. Just one day started walking!!
Now my 5 year old "did" crawl, and I currently have a 8 month old who just rolls all around, my boys never rolled like she does, but no crawling yet. They have each been completely different from each other. I would personally tell you not to worry, he may just skip it altogether. All three of mine have been different! If it overly concerns you, you could always mention it to your pediatrician.