My son is 8 months old. He rolls over from one place to the other and gets what he wants, but he hasnt started crawling yet. Of late, when sitting he has started rocking himself back and forth by stretching his arms in front on the floor and lifting his butt up as if he wants to go down on his fours, but nothing yet. Is there anything I can do to make him crawl? I do put him on his tummy and spread out toys, but he only rolls over to get whatever toy he wants. Also, do girls start crawling earlier than boys do?