I don't agree, you are the adult not your S18. When he starts to act as a go between, you say, "S, thank you for telling me this, it's not what I want from you as my S to act as a go between. Neither do I want to put you in the position of say so to your mum". In future it's ok to say "mum, please text dad" and "dad, can you contact mum by text".
She will see everything through her wayward lens.
Incidentally, I am so impressed by you.
V
Freedom is just another word for nothing left to loose. V 64, WAW