Shellie,
Sorry to hear about Merlin's condition
... we lost Mogli to KF in December, just a week before his 7th birthday. One thing I know about KF is that when their levels get very high, they feel neaseaus. Something to look for.
Has your vet mentioned giving sub-cutaneous fluids? That could make him feel better depending on the rest of his health picture and level of KF. You basically inject fluid under the skin to help flush out toxins that the kidneys can't process.
I grew up with several yorkies. From our second "run", Wicket outlived Poco. He was almost 16 or 17 and a very old little man. He was very slow and would sleep most of the time, once in a while he'd perk up for a few minutes. Anyways, long story short ... one day my mom woke up and he wouldn't eat. She just knew. So she called my sister who is studying to be a vet, and she came home so he could cross the rainbow bridge to be with Poco.
It's the toughest decision you can make. When you feel that his quality of life is deteriorating and you can look in the mirror and honestly say you're doing it more for yourself than for him ... that's when it's time to seriously consider. He'll always be with you, so letting him go isn't permenant. With Mogli, I just kept telling him to not give up on me, 'cause I wasn't going to give up on him. He was strong. When it's time you can feel it, it's the hardest thing to do.
Make sure to give him extra kisses, he knows he's very special and loved.