I also have another question in regards to my S16.

I had a very open conversation this morning with him and it seems this all has a significant impact on him, much more then I expected. He even told me his feelings are becoming numb towards his father but also to other people which is not good. He also realizes this himself. Therefore I suggested to arrange a meeting with a psychologist and he agreed.

Is this something I need to inform STBXH about or not?

The problem is STBXH currently thinks he is a great father (most of the time, when he's not drunk), from the messages he's been sending them he really doesn't have any clue how the kids are feeling towards him.