My baby is 8 months, and while I know it's completely normal for a baby to not be crawling yet, my concern is that he's really not mobile at all. He'll roll over, but that's about it. He tries to push himself with his legs, but does absolutely nothing with this upper body. He's never once pushed up onto all fours or tried to pull himself up onto anything. Anyone have any tips on how to encourage crawling/pulling up or just using the upper body in general?