I was also worried about that when my son came to me. It didn't happen, but it's a normal part of being a teenager - trying to play parents off against each other. And sadly teens can actually be easily bought! Up to a point. That is not specific to your son. At the end of the day though I don't think that would happen because he has respect for you - you described how he behaved when he was with you. And that is what talking is for - kind but firm man to man. Like - well I support you with xyz but I expect xyz as well. And give examples of famous people who manipulate etc (bad guys!) and famous people who are successful (good guys!) and how it's important how people behave. I had to learn that. Being authoritarian stopped working by 13! They rebel on principle. I had to change the way I dealt with things. Luckily my own Father helped as I remembered how he handled me as a teenager. With humour. It defused things. He'd say - well I'm not going to say no because I know you'll do it anyway so here's a tip - do it this way (or whatever).
Your son does sound like he needs help and I hope he improves now the court process is over. Re him not getting any cards - could you get them to him some other way? But this is why an online presence is important too.
I hear what you're saying but you sound like you're rejecting him already. I am the first to blame the ex for a lot of bad things she did (although I don't say that to my son), but I also accept my son didn't choose to be stuck between parents. So my advice is - don't throw the baby out with the bathwater.
And yes there is definitely a need to heal after a court process like that.
