If it's not clearly stated in the parenting agmt, I wouldn't see how DH could be charged with contempt. However, I would say that 1x/mos wouldn't ever be classified as "unusually high" - he probably couldn't win with that argument.
Our approach was that we felt it was not necessary, as my SS had no behaviorial issues warranting such treatment and, most importantly, DH was NEVER consulted, and NEVER given access or told about the "diagnosis". Our parenting agmt requests both NCP and CP to consult each other when it comes to NON-EMER medical appointments and, more imp, ELECTIVE appointments, which I feel is the case in your situation. Unless the child was/is exhibiting "symptoms" which could/would cause you to feel the appt is necessary (as our agmt states), you would NOT be responsible for paying for the visits.
Of course, this is always up to interpretation by the courts and, as we've found, just because it's written on paper doesn't mean it's worth the ink it was written in. My DH's PBFH has violated EVERY single term of the court orders and she has NEVER been held in contempt - sad, sad state of affairs.