Without a bit more context it's hard to advise the lady about how to approach the problem with her partner.
Obviously it's a very new relationship, only a few weeks, so she has very little background to the man involved.
Im not sure the word "it" means prostate cancer, just that he won't talk about his behaviour prior to them having sex. But prostate health issues must be involved or else why would she join and post on a PCa forum?
And she mentions he talks about sex all the time. We men all talk a good job when it comes to the bedroom, especially if trying to impress a new partner. As we all know having PCa might well affect the ability to get elections etc, but it doesn't switch off the bit of brain that says we still want to do the deed; the mind is willing but the body is weak kind of thing.
If Hay can add a bit of detail I'm sure some good advice will be forthcoming. Maybe the guy needs to join the forum unless he already us a member and feels he has things sorted by a brief absence from the bedroom prior to sex, which just confuses Hay. Maybe just involving her in his preparations as if it were foreplay would allay her concerns.