rocking to crawling !?

ashbash4 • Mommy to Carl and Cora; LOVE THE LIFE YOU LIVE; fuck what anyone says
So my 28 week old son just learned to how get on all fours & rock back and forth.. he throws himself forward but ends up landing on his face lol we only let him do this on the bed so he doesn't get hurt.. anyways, how long from this stage until he starts crawling !? He will be 7 months on the 29th