I truly believe that you can change a negative by using a positive and you just proved my theory.
The point is to separate yourself from the emotion. But you've also helped your son feel better about his dad. WIthout really saying it - you're letting him know that it's ok to remember the good times in a healthy way.
Your son needs someone to vent to. He feels that he CAN come to you and for a 15 year old - that is HUGE!!! He knows you know his dad like no one else so only you can understand in the same way.
I also understand how difficult this must be but if it is working for you - keep doing it.
I'm impressed with your attitude.
I am not a lawyer but when I was going through my divorce and had problems with how my ex was behaving with the kids - my lawyer told me to keep that out of the written stuff. RIght now - you need to make a business agreement. That is the written part. THe lawyer should be advised about the things that are being said to your son about you and you should keep your own documentation. But try to stick to the financial stuff because sometimes it gets muddled up when there is other stuff going on.
My lawyer told me to have my daughter tell her dad herself how she felt. In my case it was that ex was taking our severely disabled son to his house despite the fact he was not supposed to and having OW around him when he had agreed not to. My daughter, then about 15, saw her dad with her brother and OW and she just flipped out. I was incensed too but my Lawyer told me to have her and her other brother tell their dad how they felt about what he was doing. Then he knew it was not coming from me. He needed to know that THEY felt that way. He cared about how they felt, not about how I felt. And it worked. Well, at least until recently.