Just a thought.
If this is a home campaign, which it sounds like, and he is outdamaging the other characters, simply build some encounters to showcase their abilities and downplay his.
I almost totally decimated a few mid level groups with well placed low level ambushes.
Ranged attacks from a tree line drawing the characters charging through waist high grass filled with poisoned steel jaw traps and punji sticks in to a muddy clearing (dirt saturated with kerosene), when they entered the clearing they noticed the smell just in time to see torches being lobbed in to the area. By the time they could even reach the ambushers they were hurting bad.
I had hit them with about 5 different ranged ambushes.
If he is going huge, just put some encounters in areas where his space is limited so he can't be that big.
So many evil things for DMs to do to players and so little time.
Jaeru...
