Ok, he can’t say no? Suggest the solution yourself.

He is never going to nerf these guys. So suggest a solution that lets everyone get to the same power level. Figure out what kind of magical items they’ve already gotten, and then ask if every character can pick that amount of magical items, with his approval.

So for instance. Let’s say they have two legendary items each.

“Hey DM, the lack of balancing is really affecting the game at this point. Would it be easier to just let all the characters choose 2 legendary magical items each, so it becomes a bit more equal and easier for you to balance encounters?”

He can’t say no. So ask him for stuff.