He probably didn’t tell OP because what man wants to admit he needed a massive prop up from Mum and Dad?
How utterly ridiculous, a lot of people need help to get on the property ladder, if you've got parents that are easily capable of helping you, chances are that is what they will do.. it doesn't make you less of a 'man', what makes you less of a man is hiding that amount of financial investment from your wife until after you're married.
Basically the parents are like if you want to go this path you pay for it yourself we don’t want to pay to support people we aren’t related to.
They aren't paying to support OP's child, they paid for their son to get on the property ladder.. and that's what has happened, it would be the same whether OP lived their or not, their son would still need somewhere to live? Why would it matter what size house it is, he's paying the remainder of it above the £80k so why would that matter where he lives or who he lives with, if they were really that bothered about 'outsiders' staking a claim then they could have easily have drawn up a legal document to say so.. they've dropped a quarter million on their kids properties, they're obviously not shy of a bob or two.
That’s removing emotion from it and viewing it from the purely financial perspective.
If it is a purely financial perspective then they would have made it clear at the beginning that it was either a long term loan, linked to the house prices, or a gift, 'apparently' they said it was a gift, but if that gift came with unofficial stipulations that DH was aware of (it becoming repayable when selling the house to buy somewhere else for instance), then DH would be aware of it.
the fact that OP hasn't asked her DH to elaborate on any of this is highly suspicious, maybe she's assuming it's to do with her son when really it was always going to be repayable at some point and he's just failed to tell her about it?
Her version of events just don't add up at all for lots of reasons.