Cancer plus Dementia

This is my first post. I have read a lot of yours, though. My husband was diagnosed with stage iv lung cancer in November 2008. It was already in his brain and both lungs. He had whole brain radiation and 8 sessions of chemo. In July of 2009 he was hospitalized with pneumonia and renal failure. He was in the hospital for 3 weeks and during that time he had some kind of episode that left him with dementia. No one knows what caused it. It could be damage from the radiation or the cancer spreading or some kind of stroke.
But this is the crazy part. He was released to hospice care on July 27, 2009 and given weeks to live. When he came home he could not stand or walk or even feed himself. We had to use a lifting device.
But over the past eight months to everyone's surprise he has gotten incredibly stronger physically and can now even do a couple of stairs. He does not cough or wheeze or show any signs of cancer. But he has serious memory impairment. He does not even realize that he is sick. He is living far back in the past. I won't go into details, but it's a serious case of dementia. He can't be left alone because of obvious reasons. All the doctors say there is nothing they can do for him, just keep him comfortable. He has a lot of agitation and restlessness which we control with medication. It is really heartbreaking because he has been given extra time but he can't appreciate it.
I don't know whether to insist on more tests, because I know he can't get through an MRI with all of his agitation, and they tell me there is no treatment anyway so it would be just for curiousity.
He is only 62 years old. Has anyone experienced anything like this? I am having a very hard time with it.
Amelia