Aidyn never commando crawled (not even 1cm) and then at 10 months he finally started crawling in the 'normal' way. His first step was pretty much spot on 12 months, and he was walking well by 13 months.

My little sister never crawled at all, and just bum shuffled everywhere.

I also think that its perfectly normal for them to all do things differently, and at their own pace. The fact that he can get around, even if by bum-shuffle, is great, and makes me think thats just normal for him!