She really is eroding him - deskilling him, isolating him, making him emotionally dependent, closing down every opportunity so it is like he is in some invisible emotional cage. As PP said like a caged pet where she is in 100% control.
He must be v stressed, confused and feel terrified, exhausted and trapped. Even if he is in denial and doesn’t know this consciously.
He is in trauma - he is in hyper alert state - and can’t think - that’s why he can’t make a decision - she holds all the power - he is currently helpless.
And you must feel the same.
Try not to worry too much about his failed academics and apprenticeship for now - because he can pick this up later - keep focused on his emotional well-being - anything and everything to show contrast to her way of being - provide him with choices, light, fun, busy, socially connecting - you are trying to woo him back with the carrot not the stick.
He is showing his trust in you when he opens up - in a good mood he is telling you stuff … notice this and this is the time to ask questions (not judge her to him) - does that feel fair? Is that appropriate? What did you want to do? What would happen if you said “no”? Etc ….. keep supporting him and encouraging him - with 10% chipping away with challenging Qs to leave him with (you don’t need him to answer for you).
When he is in a bad mood - it’s not with you - know that - it’s the stress of existing with her. Maybe at these times he needs some TLC / some implicit communication - no words - just a hug / cup of tea …. Even though you might be angry with his bad mood - see it as him in pain preoccupied with her.
So tough for you to watch but progress is being made if he is opening up to you. Keep focusing on his MH because that is the bit that will allow him to dig himself out of this.
And keep looking after yourself because there are a few more hard yards to go yet.
Do they even have any fun anymore?