My 8.5 month old shows NO SIGNS of crawling. HELP!?

My 8.5 month old son sits up well, rolls both ways, tries to reach for things, face plants and tries to swim on the floor on occasion. He has no idea what his knees are for and does not know how to pull himself up to stand. He loves to walk all day long with me holding on to him....but is very unsteady on his feet. I have tried EVERYTHING to get him to crawl: put toys out of reach in front of him, have gone to chiropractor and did collateral movements with him, have encouraged tummy time (he always rolls off his tummy when put on it). I don't know what else to do but cannot help but compare him to other babies his age who are all over the place. HELP...what more can I do and will this just come with time???