As Chuck has pointed out above he will have no choice if approached properly and politely.
He’s carried out work that is not to an acceptable standard or compliant for the manufacturers warranty. You have to give him a chance to come back and make right. If he refuses to then take the legal route.
Looking at the quality of the install and the fact that he says it’s fine I highly doubt he gets much work and if faced with the possibility of a court case he might sort his act out.
We all make mistakes, I made a lot at college when starting out but he has to be given a reasonable chance. This is now more of a legal matter rather than a plumbing one and I’m not familiar with small claims etc thankfully.