You post an interesting question.
I would try to get him to accept some responsibility, the issue here is how much integrity does he have?
This may be a case for the Better Business Bureau or perhaps the Consumer Affairs division of the state.
I'd suggest writing him a certified letter first.
Impossible to tell if he is responsible for your most current problem but this is a reminder of why it is important hire a person that is licensed and insured.
A second opinion is always best in these situations....I'd play that card by leaving a message that you, have waited for him to respond and are now referring the problem for repair to another plumber and are trying to determine if he is responsible for your problems because of previous work done......I bet you'll hear quickly.