how to help baby crawl

my son turned 6 months on the 10th. he has been doing tummy time since he was probably 3 days old. he can sit up almost completely unassisted but he seems to be a bit top heavy and will fall if he moves his head too fast lol. he can roll over. he loves his jumper and goes crazy in it. he reaches for things and can pass them from hand to hand. so he has a lot of milestones down but i don’t know if i see him crawling anytime soon. i know all babies do things at different times but he won’t stay on his hands and knees if i put him down like that, he just slides his legs back and goes straight onto his tummy. is there anything i can do to get him on the hang of staying on all fours and eventually help him start crawling?