1st rule of GMing (in my opinion), let the players shine at what they do, and don't steal their thunder, they are the heroes.
I would definitely approach him and bring the issue with his DM style, he might not even realize what he is doing and might think he is just "helping" the party. I've played with a DM who did something similar, eventually it was just the DM in combat against himself, DMNPC vs DM is not fun for the group. Once we let him know that it wasn't fun any more we had no more issues.
