Same here - and I have a nightmare SIL who is hell-bent on comparing my lo with her children (youngest is 1 year older than my ds). So she makes me well aware that Tyson is not crawling yet (only 7.5 months tho!) when hers crawled at 5 months (yeah, right).
Anyway, don't let it worry you - try not to get into the comparing game. He will get there when he gets there.
One book I have actually says that crawling is NOT a reflection of intelligence or athletic potential. And it also says that most babies crawl between 8 and 10 months, but SOME NEVER WILL. Ask your mum what you did crawling-wise - it may give you an idea of the developmental path your lo will take.
In my case, my mum has told me that she bum-shuffled, as did both I and my older brother. So maybe my lo will do the same....