I don't think there's anything wrong with his discipline personally. If you take that aggression away from his game, then he won't be the same player.
Plus as Gimp mentioned - at the moment he seems to get booked a lot for trivial things, it's not like he's intentionally going out there to injure opponents.
He just needs to be a bit clever sometimes I think, not tackle someone when he could just block their path and protect the space. We miss him a lot when he's out, would be great if he could learn to get suspended less.
I wouldn't want to change his fundamental nature though obviously, you're right there.