To try encouraging your son to crawl, you could place his toy a short distance in his path to motivate him to crawl towards it.
It is also possible for some babies to skip crawling entirely. It is not one of the gross motor skills, like rolling over and sitting up. Crawling is simply a method for a baby to explore his/her surroundings. The baby could also be sliding on his/her tummy or shifting around on his/her bottoms to achieve the same objective.
Sounds like your son found rolling effective in getting him places. Sounds really cute! :)