If the party is actively helping him becomes more powerful and they're fine with him doing this stuff I say don't change much.

Hell one time my party did just that. I gave them a DMPC that was kind of an experiment. It was a warforged golem that was basically bare bones, but who could be upgraded with alchemy/fusion.

The party then proceeded to completely ignore themselves in exchange for turning the Golem into a walking death machine that could take a Pit Fiend on one on one.

I'm not joking, the final battle literally was "We send in the golem and wait outside the room."

5 minutes later they walked in and looted the tomb after she had smashed the entire undead army.

If the party wants to just smash stuff, sometimes making one character powerful enough to do a lot of the work isn't a bad idea.

But again. If the other party members are complaining ask him to tone it back a bit or do some kind of counter.