Triple check character math. Pathfinder is a game that is VERY easy to make a mistake that can be game breaking.
My best advice is to NEVER try and get around pc choices. By and large you can't make a unhittable character without being seriously short in other areas. This is why doing tons of damage AND being unhittable sets off so many alarm bells.Check the math!!
If the mobs wail away at the fighter and can't hit him..have them ignore him,knock him into pits or whatever is handy.
I just think bypassing player strong points in order to "Get Them" should be discouraged.
Now since it's bothering other players IF the math turns out to be right"Don't think it will based on what I have seen in games" then you can simply ask the player to change some things about the character.
Before I was that drastic however I would simply make sure the math is right and add fun encounters for the other guys. Might be its only a issue because you have been running more encounters that stress that players specialty instead of the others?