since they started to sleep together there's been a big shift in the relationship - the first couple of months she didn't want to, and he respected that, all good, and he seemed nice and charming etc etc. As soon as they start sleeping together this whole other side of him came out - the real him I guess. It was very noticeable. Does anyone else notice this with teen relationships. I've seen it so often with dd and her friends. It's like the boys think they own the girl after they've slept together. I'm not being old fashioned I don't think I've just found it really noticeable.
You're not being old fashioned & it's not just a teen thing.
Abusers tend to escalate as the relationship progresses through the usual 'markers' - eg engagement, marriage, cohabitation, & certainly pregnancy/childbirth.
With teens, they're usually only dating, so the big demarcation is when they start sleeping together. Yes - it's about his perceived ownership of her - but it's all rooted in the cycle of abuse (Idealise / Devalue /Discard). With careful probing you will be able to discover that he rewards her for compliance - Idealise - that phase lasts less & less time each cycle - he moves into Devalue with his idiotic instruction about using the word 'valid' to shut down DD's feelings - he will soon move into Discard where he punishes her for non-compliance,, or invents new 'offences' he can berate her for. Discard can be the actual ending of the relationship (usually temporary, as a manipulation, until he Hoovers her back in again with fake contrition & promises), or stonewalling, withdrawing, verbal abuse ...
DD then either sees sense & walks away, or is still enmeshed with him, thinks it's all her fault, & appeases him. In which case he begins again with Idealise.
This cycle plays out time & time again.
This content is likely to be 'too rich' for DD to digest right now - she's not going to welcome any notion that her b/f is controlling. We also don't know if hes personality-disordered or ... just a knobber (thanks PP!) so she would probably reject the descriptors of abuser behaviour in the link, as she will not want to consider that her b/f is abusive - let alone seriously dysfunctional.
But he could be - so, as background for you, to help you watch out for the usual indicators, you might find the site helpful -
outofthefog.website/toolbox-intro