It was "phone sex" and T's, especially men, aren't generally supposed to engage in that sort of thing, especially with someone calling them for help (and you were, to keep in touch with your son's condition, so he was more a family therapist than just your son's therapist)?

Another way to think about it; what if it had happened that way IRL? Would you think it unethical then? There's not much difference just because he's on the phone and can't "see" you. He was "manipulating" you and getting his jollies or whatever. You "had" to call him because he is where your son is so he has "power" in the relationship between you -- would be like secretaries sleeping with their boss to get a promotion, that old sort of problem; you "could" have been doing what you did to get better treatment for your son, theoretically. I don't think you were doing that and I don't think it was unethical or even necessarily wrong on your part (since it seems to have helped you a bit to be more "daring," something you presumably want?) but it was definately wrong on his part, especially since he initiated it.