While I 100% agree with the thread, it’s also worth remembering that DP has been blind-sided by this as well. He’s way out of line not pausing the sale himself, though.
OP you’re feeling that you’re unable to call him out on this because you’re own DC will be coming and going when you move in together, but this should still have been discussed with you first.
Despite feeling that he’s been surprised by this, I’d still pause the sale TODAY and plan a mighty long chat with him. Having the son move in full time is totally a different setup to having your own DC use your home as a base while they live their own lives. How do they feel about this man moving in?
Also, it’s so disrespectful to dismiss the other Mum has constantly “moaning” and having a go - I’d bet that there are some hefty and significant reasons why she’s unhappy. You are going to inherit a lazy, messy teenager - think of you cleaning his piss-stains off your toilet seat for starters, never mind the sticky smell of weed.
A word of caution - I know someone closely with this same situation and the son still lives with them aged 28. Smoking weed was recreational but is now so firmly entrenched that their only motivation is doing their work shift and returning home to get stoned. Zero help in the house, doesn’t contribute to cooking and cleaning. She tried not making his food but he instead started ordering deliveroos which arrive at all hours - they’ve been “saving for a deposit” ever since they arrived but I’m highly sceptical. Oh and the joint smoking down the garden lasted a few weeks until it became “too cold/too wet” and “ I’ll smoke by my open window “ and now they just lay in bed gaming, getting stoned and drinking cans. Once a month, she has to go in and remove bags and bags of cans and filthy crockery and her home (which is beautiful) now stinks of weed and I no longer go there. It sticks to the walls worse than nicotine.
Her partner is hopelessly ineffective and my dear friend is stuck.
OP, pause the sale and have the talk. He should never have sprung this on you and should in fact have paused the sale himself. You are about to walk into a situation where you take on the other Mother’s problems and stress instead of upgrading your own life. No thanks.
If it was me, I’d 💯 pull out and live separately until his son gets his own place. Let Dad sort him out otherwise he’ll be YOUR problem. It’s the only way to protect yourself in my opinion.