I've been put in a bit of a situation

[Ohfgsjon]

I've been with my oh for 10 years. He has 2 adult sons. The youngest is lovely, the eldest one is more challenging. He's been in therapy for years, has a bad temper, is something of a misogynist but can also be perfectly nice. His father is very wealthy and the boys went to private school and have had an extremely privileged life. They both have very good jobs. Eldest has never had a serious girlfriend, just short lived flings. He occasionally stays at mine for long weekends for a change of scenery and when his father is working away. He has a difficult relationship with his mother and they are very low contact. He can be perfectly ok with me and has confided in me about all kinds and we've chatted etc. He can also be moody, sullen and brooding and has been verbally aggressive and cutting to me. Hes very big and has a large physical presence. When I've raised his attitude with his Dad, he's just said he doesn't mean it and he's a good lad really. A couple of weeks ago though, I found out (via a family member) that he'd actually pulled a knife on a female relative and threatened her with it. I'm really quite upset that I've been put in a position to be left alone with this man and my oh didn't see fit to tell me (He knew all about it). How on earth do I approach this with my oh and is it ott to insist that I'm never left alone with him?

I would say you tell your OH you have found out about the incident with the knife and why hasn't he told you about it?
And because of it, SS isn't welcome to stay with you anymore. I can think of no acceptable scenario short of her pulling a gun on him where this would be in the least bit ok.

If he kicks off about how it was a one off and his son is lovely, stand your ground. I know of a few parents who still maintain their darling sons are lovely, when they are total shitbags. They think that because they don't run around doing these shitty things ALL the time, and sometimes do normal nice things that they are decent blokes. They are not, because decent blokes don't pull knives on women - or anyone. Ever.

Say to your OH you don't feel safe. And hopefully he'll put your not feeling safe above simply not upsetting his son. And tell him thats not because of anything you have done, you've been more than accommodating, its because of something his son has done. He can hardly expect to pull a knife on someone and everyone just say -oh yeah, but hes a really nice bloke. FFS

Agree with PP, tell your husband you’ve found out and are concerned and upset that he didn’t tell you before. He’s potentially put you at risk and that is not okay. I would not allow the son to stay with you alone anymore but if that’s easier said than done and there are occasions that he will be in your home then cameras, cameras and more cameras.

Of course you can say it’s no longer ok for DS to stay with you. But does that solve the problem? Your OH kept this from you. Is he minimising it? Is it time to re-evaluate the relationship? Seems like a serious breach of trust to me.

He occasionally stays at mine for long weekends for a change of scenery and when his father is working away.

Is it your house, solely in your name - or is it a shared tenancy.mortgage?

If it's solely your house - you just tell it like it is. No more staying with you for this young man.
If not - take @Bookworm20's advice - it's solid.

That should sort it short term.
Longer term - over to you, but Id be questioning the validity of a relationship where my OH not only withheld vital info from me like this, but willingly exposed me to danger.

You might want to have an enlightening conversation with him, starting with "& how have you responded to the female relative he pulled a knife on? How many relatives are you prepared to put in situations where they are alone with a man who has form for pulling knives on women? Why did you think it was ok not to tell me? What are you going to say to the police if you come home to me injured or dead?"

I'm so sorry for the shock & dismay you must be feeling OP. Your OH has been a prize shit over this, I would be very angry in your shoes. He seems to want to protect his son's reputation more than he wants to protect YOU. If it were solely my house, I'd be asking him to leave. The lying & lack of care is astonishing.