Well, to be perfectly honest, it might also have something to do with the fact that dustox are naturally more aggressive than beautifly (which isn’t to say that beautifly aren’t more vicious, but that’s another story). So that should at the very least explain the attacks.
That having been said, the best thing to do is to help your dustox acclimate to his new form. It wouldn’t hurt to apologize for promising that he would evolve into a beautifly, but your main focus should otherwise be on convincing him that dustox aren’t so bad. While you can teach him the positive aspects of being a dustox, your lessons will definitely stick if you train with him. Set aside an extra hour to work with him one-on-one on abilities only he can learn (Light Screen, Moonlight, Psybeam, and Venoshock, for a few examples). Devise a battle style that suits him, and make your training revolve around that. Highlight his importance on your team and power as an individual until he understands how his body and abilities actually work. The closer he gets to this point, the more he’ll become proud of what he can do.
All you’ll need to do from there is keep him from attacking your beautifly out of pride, but the secret there is having the two team up and work together through battles against other trainers or team building exercises. Or through sheer positive reinforcement by bribing him not to attack your beautifly. Whichever you prefer.
Best of luck!