At the same time here, as mentioned above, it's not at all clear that letting a dying kid have sex once in his life is wrong considering the circumstances. The wrong is generally considered to be in the (in this case nonexistant) long-term consequences of the behavior. If the kid wanted to smoke a pack of cigarettes before dying he should be able to do that too.
As for the legal issues, the age of consent in Britain is 16.
Basically you are saying that at 15, you or someone else knows better than the parents who are morally and legally responsible and to that I say ... hogwash.
I've made my position clear and as far as I am concerned the decision must remain with the parents, not some do gooders reaching in from the outside.
WRT Doctor Client privelege ... I amke it clear to every doctor my kids go to that the business is contingent on full disclosure to us parents until they are adults. Sorry of that ruffles you ... but as the one legally, morally and financially responsible ... that's the way it is. I would never try and propose that at your house it should be my way ... but for you or someone else to say that in such circumstances this parental right "thing" needs to be bent is morally indefensable.
Like I said though, I have made my point and others can read and determine for themselves.