I'm so sorry...he sounds like a very special little boy...You have my sincere condolences - there is nothing sadder then the day it's that time.
Just to help a little - I was with my husbands cocker and while I cried and lost all my emotions there - but so you know - she did go so peacefully...
later I took alot of comfort in that but it's so very hard no matter what. At 18 I would have to think he may not want to fight anymore ...that's a very long lifespan for a yorkie and a tribute to his good care ...Bless his heart - he sounds like such a joy.