www.nspcc.org.uk/what-is-child-abuse/types-of-abuse/emotional-abuse/#effects
Have a look at the link above which outlines the signs / impact of emotional abuse on the behaviour, emotional and mental health of the recipient.
This boy is displaying all of the effects.
He is isolated, reactive, failing academically and socially and appears emotionally unstable, depressed and anxious.
This relationship is totally dysfunctional FOR HIM - it has destroyed all these area of his life whereas the GF is flying.
He’s not holding HER back - but she / the relationship has eroded him.
She gets her needs met by emotionally discharging her internal anxiety by being domineering and controlling.
Sounds like GF parents are delighted that he is in the frame to absorb her nonsense which was previously coming their way. He’s a convenient buffer - she’s doing well - they have zero concern that his MH, family life, academics, sport, social life, work, etc have diminished - as long as he keeps her happy they don’t care - he is expendable.
The OP doesn’t jump to their drama - she does the opposite - she is remaining centred, calm and compassionate as well as applying appropriate challenge for someone emotionally unwell because she can see how his sense of self worth has deteriorated in this dysfunctional relationship.
If you pan back you see a young man who’s life is spiralling, who’s consistent emotional state is one of FEAR - paralysis, indecision, exhaustion - that’s what being in an abusive relationship does to you.
The end game is that this relationship needs to be over.
It’s then a decision as to how that’s achieved. Ever DA service approaches this by supporting and encouraging the victim - not bulldozing in with an authoritarian high handed approach.
I suppose that @Marni83 your position is that the relationship is irrelevant - that he is just a feckless teen. If that were the case he would be out and about with his mates, hair raising and being reckless - that’s exactly what’s NOT happening here. The clues are his withdrawal, depression, reactivity, his fear and constant placating of her.