This is a problem I ran into over the years and I wish there was an easy button fix. The issue is what works for one kids doesn't for another. Everyone ultimately has their own swing/mechanics/timing/etc.... I found the most success with teaching oppo by focusing primarily on the mental side of hitting. Instead of saying hit the ball to RC gap, I would emphasize DRIVING the ball to RC gap (RHB obviously). With this in mind, then all the kid needs to do is recognize pitch location and timing.
But if your looking for a drill, the one that help my oldest the most was the drop bat drill. Basically he would swing the bat extremely choked up with the emphasis on not hooking his rib cage with the handle. Forgot where I picked this up over the years, but it really helped him get through the ball, instead of around it.
(with this all this said, I don't necessarily promote oppo hitting in all situations. I want the kid to feel comfortable when he gets ahead in the count and look to do damage)