To think DH is losing the plot re: DS’s new girlfriend??

Well thanks for the education, I’ll start using that term.

Your husband sounds like a fucking lunatic.

You are definitely right to be concerned.
All logical possibilities are sounding pretty grim and because of this I'd have my fingers crossed it's something medical that a GP can put right. Good luck 👍

There must have been other strange behaviour from him. This is insane.

He’s seen her on a dating site.

Your son and his girlfriend are still children, minors, at age 17. No one is going to investigate a child and your husband could get into trouble with this. I would explain this to your husband and make sure he stops.

Grandson age 19, meets "lovely" girl at college. Girl is 17, grandson has severe dyscalculia!
Lovely girl "helps" the boy to access his Child ISA. My grandson buys the girl everything she asks for, she also "helps" him to transfer money into her account. Net result out of an ISA worth £21.5k within 2 months only £6000 is left, £8000 directly transferred into lovely girl's account.
After meeting her mother, it turns out she has done similar to at least 2 other boys. The mother wrings her hands " what can I do?" she simpers.
The police are aware and say my grandson was subject to (all kinds of) abuse. My grandson won't press charges though, he feels he's a stupid victim and everyone will laugh at him. He's fallen out with his parents over this. He's currently living with me.
Yep, everyone thought she was lovely, everyone believed the lies she told, no one could imagine that she was a wrong'un.

There’s clearly far more to this…

How the hell would I know ?! These are all just suggestions as to what it could be. He has mentioned a PI so I offered another reason why he could be going off on one.

He could be her father]
He could have had an affair with the mother
She could be a dealer he has bought drugs from
She could be a sex worker and he is/was a punter

This all pure conjecture from everyone. The only person who actually knows is him!

What's being a customer got to do with being the girls father or having been with her mother though?

OP, I have a funny feeling this is connected to the girls mum, not the girl herself.

Is there a chance your DP had an affair in the past with the girls mum?

No idea! I wonder if the "something" is perhaps illegal (drugs were mentioned by PP, could be something like that) so a PI could "uncover" that so he wouldnt have to tell OP that actually he was a customer. Or if she is a sex worker say.

This

So what's the purpose then of a PI in those circumstances? What would the PI uncover that could then be shown to OP?

Oh thank goodness someone has said this! It was my immediate thought but I was feeling bad for thinking it.

Also, I agree that he already knows something but its something that would make him look bad if you found out.

My feelings are either he thinks he is the girls father, or he has had some other relationship with the mother, probably while in a relationship with you given that the kids are the same age.

A white social worker told me off for calling my daughter mixed race. She told I should say "dual heritage". I asked what did that prove, because unless someones parents are brother and sister, everyone is dual heritage. She never mentioned it again.

I think the girl being mixed race is a huge red herring. Racism doesn’t usually present as stalking people on the internet obsessively. Plus, I’d be very surprised if you’ve been together 20 odd years and there’s been zero previous suggestion of racism. Racists are desperate to talk about why they’re hard done by and oppressed and all that bollocks. They don’t tend to sit on their hands and bite their tongue for 20yrs.

No, this is almost certainly sex related. Could be the girl, could be the mum. If it’s the girl it could be anything from him finding her attractive, having an erotic dream about her, finding himself thinking of her when he masturbates etc to him having made a pass at her previously. The latter could be why she was so shy, the former could be why he’s freaking out. If it’s the mum, it’s either a ONS previously and he thinks he’s the girl’s dad or a more recent ONS or affair that he’s now freaking out about as it may all come out.

Of course, the third option is that your husband is having a breakdown but I suspect there’d be other signs if this was the case.

People jumping onto he might have MH issues,
sounds to me like he’s racist.

I agree, he knows something about the family/mum/gf and he wants the PI to 'discover' it.

I think is OTT rection to your questions is all about covering up the fact that he KNOWS what he is asking is unreasonable.

That reminds me of an episode of House which had that exact plot even down to the racial element

Oh well, the thread is imploding slightly.

OP I think to be honest that the speculation in some of the replies on here are utterly over the top and are quite ridiculous.

I don’t think it overly weird that he has looked for a bit of background on the girl, she will soon perhaps be staying in your home and so I think that’s fair enough. What I do find really odd is the private detective conversation, has he been watching too many true crime programmes that have affected his sense of reason!

Personally I wouldn’t tell your son, if you do so, I think not only will this thread implode, but so will your life as you know it. I would tread very carefully for a while and see where the conversation goes over the next few days.

Is she secretly his child?

There's something seriously wrong with him, OP. That's fully batshit behaviour and fucking creepy, stalking behaviour of the girl and her family. Gross.

We’d ended the conversation over 20 minutes before you wrote that comment. If anyone is ‘goading’ with daft non-issues, it’s you.