If telling him to try other chars did not work, well, then maybe you can try others, and maybe you'll be even better twith them than what you are with your actuals, or maybe you'll give him more challenge since he might not know how you're going to react with that other character, that worked with me, long ago i used to play only C.Falcon, and my bro always beaten me, he still played, but i was the one who was getting bored, so i tryed different characters, i started using Fox, Mario, Falco... and i saw how he thought i was going to do something i'd normally do with Falcon, but i did something else and started to beat him, then i started using even more characters, and at this time i use: Marth, Falcon, Gdorf, Fox, Falco, Mario and Luigi(Clones are fun
), i might not be great with all of them, but im not a fully competitive pplayer, so it doesnt matter, since i can change from Fox to Mario from one match to another and have completly different results, wither i win or lose
, so yeah you might want to try different character to give him even another difficulty besides just praticing with you main(s).