Speaking from my own experience ditching a nasty rolling habit, I don't think it's necessarily such a bad idea to say "stop rolling, period." Your play will suffer greatly at first obviously and chances are you will lose matches but that's not important. Completely removing rolls from your game forces you to experiment with alternatives in a variety of situations, and in turn requires that you actually think and examine the situation before you act. For me, anyway, that was the root of my rolling problem: not so much that I thought rolling was safe when it wasn't, but that rolling had become an unconscious reflex/ knee-jerk response to certain scenarios. I had a friend who played peach and loved to punish rolls with dsmashes. Naturally this was quite predictable, I could frequently see it coming well in advance, yet I STILL kept rolling into his dsmash exactly the same way again and again because in the heat of the moment the pressure to act quickly had me relying on reflex rather than thought. To make a change, I needed to ditch the play-to-win mindset and focus solely on improving. So yeah, I just stopped caring about the match outcomes and resolved to stop rolling entirely. Once I was comfortable enough with jumping OoS, dropping and running, etc, that I didn't feel the impulse to roll anymore, I allowed myself to start rolling again in addition to all the other options I now had at my disposal. At that point not only was I less predictable, I was an all around better player because I was more accustomed to playing with my brain instead of my gut.
I'm rambling, but the point is: if you want him to improve, put aside the notion of playing to win. Slow the pace a bit and give him opportunities to try new things. Competitive situations are helpful for learning how to apply familiar technique, but terrible for learning new technique.