Contractor wants $350 if the mold is not his fault

[William Levin]

We had a contractor build a new master bathroom but were disappointed with the quality of the work and did not let him bid on any more work on the house. Today I discovered mold on the wall of the adjoining room, directly behind where the new shower is. I texted him a picture of the mold asking him how he planned to address it and he responded by saying that he would come look at it but if the mold had nothing to do with the shower he installed, he would charge me a $350 trip fee. We didn't part on good terms and he knows he is not getting any more work from me so I feel that he will do anything he can to avoid responsibility.

I'm confident the mold is from the shower. How should I proceed? Should I agree to his terms and have the Go-Pro running when he cuts into the wall, or should I have an independent inspector come out and inspect not just this leak, but all the other work he did? I'm sure the inspector will find other issues such as puddling on the shower pan, unlevel floors, excessive tile lippage, etc that we could use against the contractor if we had to. I really just want to move on and I'd fix it myself if I thought it was just a simple leak, but I don't want to cut into the wall and discover there is a bigger mess and the contractor have an out by not being present when I opened the wall.