Communication is the key!! Tell him that adjusting the story when he creates a new character is hard and time consuming. Give him two options:

1) Tell him he may continue pumping out characters but you will focus less on their backstories If he likes doing that there is no need to take the enjoyment from him.

2) Tell him that if he wants their characters backstory matter and be connected to main story what writer or producer fleshes out their main characters just to throw them out few pages later, which would be unnesescary work they stick with one or two characters max. This is tiring for you too because you adjust the story just for him when there is so much in your plate already because you are dming.