My experience from working in a nursery and looking after SEN children is that skipping crawling is SOMETIMES a sign of a developmental issue, 2 of the children i know bum shuffled etc instead, have both been assessed as being mildly autistic though they both have different issues. But i know other children who didn't crawl and are no different than ones who did, so sometimes it can be part of a bigger problem but there would be other things going on as well. Crawling is meant to make lots of new connections in the brain and skipping it means they skip that phase of learning but tbh..if it was my child i wouldn't be worried they all find their own ways of doing things and as you said he doesn't like being on his tummy so thats that he'l do what he wants x x