Well, I had something typed out, but my browser crashed. I was able to recreate most of it, but it took some extra time. Sorry.
That's even worse than I thought. I'm not sure how much I can assist here. I'll try, though. Correct me if I'm wrong about this, but >>>>>> he sounds like the type to not take advice well. <<<<<< If that's the case, I'm not sure how much you can do.
Try having him switch characters, preferably to one you or another who can help knows well. I see you have Dedede as your main, try him. If he doesn't want to switch, it's not super important, it's just easier on you. Go through the moves of the character, explain what they do, and how they're useful in different scenarios. Note the speed of the attacks, both for how long until the hitbox is active, and how long after the hitbox is gone until another action can be taken. Also note the areas the attacks hit. Explain that these affect when moves are best for different scenarios, and that you need to be aware of, and use all the moves that a character possesses. Also emphasize that repeated move use becomes very predictable.
That's a start, at least. There's definitely more, but it's a start.