Contractor installed three piece tub surround straight to studs with no backing. He finished the job and didn't say anything or show me how much the surround pushed in. I discovered this when I was cleaning the surround and it dented in between the studs. He said he would have to take everything including the tub, vanity, etc. out and redrywall the entire bathroom to fix this and would charge me a large amount for this. My research (talking to other contractors and plumbers) tells me backboard is needed to stabilize the surround and that what he is telling me is not entirely true.

The first thing I would do is find another contractor. Get 3 estimates and go from there.

If you have a contract,check the writing and see how the work was to be done. If not done according to contract sue him and notify the BBB.

The tub surround needs to be removed and concrete (wonder board) needs to be installed and sealed before a new surround can be put in.

I'm in the process of having similar work done. Placing a tub surround directly against bare studs does not sound like it would be up to code. So I would check with the local building dept. If it not up to code, then I would hire a new contractor and so the original one for a refund of what you paid him.