he was tested for fanconi by the breeder when he was a puppy. he can't suddenly test positive now can he?
Well that depends how he was tested. If he was only strip tested as a puppy, he could have developed fanconi now as fanconi is usually a later onset disease - 2yrs or older (usually around 5yrs is more normal I think, but I was originally told to start testing after 2).
If he was DNA tested and you have verified results that he is a clear, then no he wouldn't develop Fanconi. If he was DNA tested and the result was Carrier, then the answer would be "most likely" no. I believe there is one very uncommon chance with the DNA test with the carrier result (if he is a fanconi carrier, and both his parents were carriers).
You can verify the results of a DNA test yourself on offa.org.
If he is 3, I don't believe the DNA test was available when he was born - when was he tested? I adopted one of my adult basenjis in early summer 2007 and the DNA test had just become available.