Should I tell my future daughter in law?

Thank you all again.
I will encourage him to 'come clean' to her on basis that full disclosure is the best start to a (hopefully) life-long realtionship.
What a great support you all are, helping me to decide what to do. Thank you again.

Gosh no - do not take it upon yourself to tell her - this would be quite wrong.

If he asks your opinion then tell him that a lifelong relationship needs to be based on trust and honesty.

I have a relative who had a major psychotic episode and was sectioned. Did well on drugs and eventually able to stop them. Future spouse told and they are happily married.

I don't think you ought to tell her, although I do understand your concern. However, your son doesn't want her to know, so I am afraid if you can't persuade him to tell her, there is nothing you can do.

If your DIL complains later that you didn't tell her, you will just need to explain that you didn't feel you could breach your son's confidence.

Hope everything turns out all right.

Did your son have medical treatment when he had these problems?
If so, perhaps ask his doctor in confidence about the likelihood of recurrence, inheritance etc.
Then discuss with your son. If anyone should tell her , it's him.

I would say it's not your information to divulge, but I would also have a frank talk with your son and explain that he is laying a very shaky foundation for a relationship if he is keeping secrets from her. It seems he doesn't trust her to understand his past.

My parents had genetic counselling before they started a family because DM had a history of mental illness in her family.
Luckily? the counsellor advised that she probably wasn't a carrier and as DF had no such history it would be safe to have children.
Did they have enough knowledge back in the 1940s to offer such advice?
In your case I think I would encourage your son to tell her. A relationship should be built on trust but it certainly is not your job.
I wish them both well.