I am used to exdh doing the right thing all the time, he is a good guy and has always done the right thing his whole life. This is all so out of character for him
OP, it sounds to me as though he's done exactly the right thing all along. He stood by you when you got pregnant (a pregnancy which you've admitted you tricked him into), he married you, but ultimately he wasn't happy. It sounds as though he even left your marriage before entering into a relationship with his now wife, when he realised he had feelings for her perhaps. If so, then that is not an affair, and she, if that is correct, was not an OW.
Even if she was the OW, you have done yourself no favours at all. You have behaved irrationally, nastily and out of spite and a wish for vengeance. Your decision not to allow your son to go abroad for a holiday is so sad, and the only loser is your son.
Your ex, it sounds, has managed to pay for his child on time, in full (with extra), sees his child regularly and is a good dad. This has all been achieved in spite of your best efforts to prevent this. Good on him. I am not surprised that he's arranged his pick up and drop off places and times so that he doesn't have to see you. I think he's behaved extremely well in difficult circumstances.
I think that you really need to learn to love yourself, and build up your self esteem. At the moment you're allowing all this to define you, and to prevent you living your life happily. Behaving the way you have (and continue to do so, assuming the holiday request is recent) is damaging to you, and to your DS.
Try a different sort of counselling perhaps - I have friends who have used NLP - neuro-linguistic programming - which may help you to reprogramme your mind, and change the way you think and view things.
If you think I don't know what I'm talking about, my exH left me out of the blue one weekday night, for an OW he'd been seeing for a matter of weeks, leaving me with 2 children under 3. They too are now married and have further children. My situation sounds similar in that he said he hadn't really loved me, and had been unhappy. I didn't trick him into any babies though, they were well and truly planned. However, the difference is in how you deal with things. Very quickly I realised that the best revenge was not to be angry or bitter, but to live well and to the full. At least, it started as "revenge", but actually soon became just how I felt. Now, a decade on, I am remarried to a wonderful man, and I am very happy. But this could not have happened if I'd clung to false hope and continued trying to make his life difficult.
I too was 35, so please don't think this means you are all "washed up".