I recall this statement as well. So, I did a cursory search and found this. Not the exact phrase (wherever that may reside) but a statement that implies that his health was in serious jeopardy and we logically imply that this also means failing health.
This wasn't quite what I had recalled so I went to do a (cursory, also) search anyway - and i think it was this sentence that had led me to the conclusion we all seem to have arrived at:
I have told the FBI and others over and over: Duane was desperate - a man diagnosed with a disease that was considered to be fatal at that time and with a painful life on rudimentary dialysis. He did not want to die the way his mother did - and he made his choice --- his one last chance to prove he was not a Mouse or a misfit who had to live up to the image of his brother.