So the OP was already dating the father, after having been friends for years.
Then the son decides to ask out the Dd of his mums partner, without a single thought for how awkward it’s going to be for everyone, especially if she says no.
She says no . But he doesn’t take that for an answer, he asks her again, at least once, perhaps several times. He doesn’t think about how difficult that’s going to be if she says no again, which she does.
Only then does he think “ Oops , this is awkward having the dad around “.
So he decides the solution is to guilt trip his mum into ending HER relationship. The first one she’s had in 6 years of devoting herself to bringing up her son after losing her husband.
Nice.
Some 17 year olds might deal with this by making themselves scarce when mums partner is around. You know, like getting a job at weekends, volunteering , joining a sports team, going to the gym, going out to their mates for the day or having a sleepover at their uncles. Or even just staying in their bedroom playing video games like most teenage boys.
But no. This lad decides the way to deal with his own embarrassment is to lie to his mum and try to destroy her happiness.
It’s clearly nothing to do with the house being “ just for him and his mum “. Because if this girl ( or any other ) says yes to dating him, I assume he’d expect her to come to his house , or perhaps even stay over. Or is he proposing to stay single forever, just him and his mum? Of course he’s not.
It’s a very sad reflection on this young man and a very poor way for him to treat his mother. He is not a child - he’s old enough to drive, leave school, get a job or go to college or university. And he’s old enough to see that his mother is entitled to her own life and some happiness and companionship.
Im also a single mum with a son of the same age and another one who is away at uni. My 17 yea old is happy for me to go out and have a social life, the same as he has. He would feel guilty if I was sitting home alone every night waiting for him to get in. And he would worry about me when he moves away to university in 7 months.