What Causes Foul Taste In The Mouth Of A 72-year-old Male?

My partner has developed a very strange ailment. He is unable to eat food or drink because of a foul taste in his mouth, that makes everything taste disgusting, to the point of gagging sometimes. This developed about three weeks ago, and he has now lost 20 lbs. He can manage to drink small amounts of meal replacement preparations, but only because they can be swallowed down quickly. He is starving and becoming weaker. Just before this happened, he felt like he might be getting a flu. That didn t happen but this condition did. He is 72 and is currently on medication following successful treatment for prostrate cancer (Lupron, which has a number of nasty side-effects, as you will know). His doctor has no idea. Please let me know what you suggest. Anne Thornton-Hill, Balderson, Ontario