you can time travel 2 days in the future with isabelle clock, enter your town, save and quit, and come back to your regular town. Go and walk arround your town till someone ping you, if it is not about moving, you can talk to different villagers to know who wants to move, then look for him/her and if it is Ken, good, tell him the negative answer, like "good luck" or "good for you" or "bye". If it is another villager you want him/her to stay, just say "don't leave me" or something like that, and then repeat the process till Ken wants to move.
Some tips:
Time travel is dangerous for the perfect town score, because if you don't have the beautiful town ordenance, your flowers may wither and there may be weeds all over your town. So I recomend you to have the beautiful town ordenance active in order to avoid this.
Some villagers, may change their minds when they want to move, even when you say the good bye answer, so, try resetting the game, just press the home button and quit without save, enter your town again and wait till the villager ping you again to tell you he/she wants to move, keep trying till they give you the "thank you" answer, and then just wait till he/she leaves.
If you resett, keep in mind that your progress will be lost till the last time you saved, so, try to not do anything important that day, or save before you talk to Ken.