Even a jam requires a good “rock” solid drumming foundation. It doesn’t have to be more about the “roll” improvisation.
Years ago when Phil Collins was popular, charity concert promoters would bring him in to guest on songs. The problem is, Phil didn’t always learn the songs. This really ticked off Jimmy Page off, enough was enough, and he let Phil have it in an interview. It’s important to learn the song!
Why not try bringing the jam to the drummer rather have the drummer bring the rock’n’roll to the song. Encourage him learn and play just one rock song well. Build some nice fills around that for jamming. Then have others play around that. Then learn another rock song and eventually build up a repertoire of song, one step at a time. As Phil demonstrated, you can’t always get away with faking it.
There’s an entertaining Levon Helm drumming video at Homespun where he talks about his approach to playing different styles.