When your child has a problem with the other parent, resist the impulse to assume the role of fixer.
Your interference in a relationship you don’t control has little chance of reforming the coParent. The coParent will likely resent it that you are sticking your nose into the other parent’s relationship with the child.
A more appropriate role is to work within your own relationship as a “coach” to help your child develop his own plan, as is age appropriate, to engage with the coParent in constructive ways. Many techniques can be modeled in the child’s relationship with the parents.
Julie Ross and Judy Corcoran dedicated a portion of their book Joint Custody with A Jerk to advice on coaching your child when the child has a problem with the other parent. It is well worth reading.