You should have been given a contact for a clinical nurse specialist (maybe a Macmillan nurse). If not, call the oncology department and ask to speak with one to have a chat about your husband's treatment. I think you might be mistaken about him being incurable, because he's been treated as though he's curable, but equally there's other inconsistent information, and it's difficult to know which parts are wrong, and which parts are correct, so I don't want to give you false hope. There are cases where incurable patients have been offered radiotherapy or prostatectomy as a means to extend life, but it's just not clear if this might apply here, and I think HDR Boost is a most unlikely treatment for such a case.
You should absolutely know and understand what treatment path he's on. He may be incurable for some other reason we don't know. The life expectancy given in your original post again looks inconsistent with the rest of the post.
Something else that might help is to contact your local prostate cancer support group and ask for help understanding the letters you've had over the course of his diagnosis and treatment. They might also be able to sit in on a call with a clinical nurse specialist or consultant, to listen and perhaps ask questions on your behalf, and then explain it more clearly afterwards, without the constraint of a 10 minute appointment slot.