In addition to rolling, he's recently been army crawling i.e., pulling himself around on his arms while his legs remain extended behind him.
And THEN, he started full-on crawling yesterday. He's not at all exclusively crawling; it's more like it's yet another mode of mobility that he tries out in between rolling and army crawling. But he's getting the hang of it and it's really charming to watch. View VIDEO here.
So we can check "crawling" off of the milestone list. Hooray!