Oh hon, that is so sad. I do believe they know when their time is up. I remember my parent's cat, who hated any form of attention at all, came up to me one day and snuggled. I said to Mum "I think he's dying." He lost the ability to walk the next day and we had to send him home. An autopsy showed he was full of cancer - it was terribly sad.
If your boy is 16 he has had a great innings hon. And I'm sure you have given him a wonderful home, and that he knows you love him so very much. I know it's so heartbreaking but he has earned his rest. Just give him all the love you can while you still have him.