My son, was 13 when I was where you are now.
He took the break up extremely hard and ex's mantra of 'I'm not leaving you I'm leaving your mum' didn't wash at all. This was because ex didn't make the effort to see him on a regular basis. Also, on a basic level son saw that he had left him, simply because he left our home.
It could have worked out, but ex was too distracted by his new life, his OW was very much his priority. He even put OW's sons before his own. There was a time when he could have taken son to a medical appt but choose to spend time with one of OW's sons instead.
My son became terribly withdrawn, and the following couple of years were hard, he had counselling.
Five years later, and the 'honeymoon' period is long over for ex, and he has finally made an effort to reconnect with our son of 19. It's too late, son doesn't want to know. Ex, in his typical fashion, has accused me of driving a wedge between them. I haven't. He did this all on his own.
I can honestly say that the indifference towards his three kids hurt me far more than his treatment of me.
Tell your son's new Head of Year, pastoral team the situation, they need to know.
And you know I said when I posted at the start of your thread that I thought there was an OW? I am now certain of this.
hugs