This being said. Getting Felv is not easy. I have had it in my home twice,
and my resident, well established cats have not gotten it. My younger
(under a year) one did. However, he only ever tested positive, he never
came down with symptoms, and with some loving care, high quality food, and
high doses of vitamin C, he now tests negative.

Just so you know your cat never had it, he tested positive because he
was showing exposure to it but managed to fight it off and never got the
virus which is why he is now negative. There is no cure for Felv so he
could never have had the virus.

As for when to test you can't really test kittens until their veins are
big enough to get blood out of with a needle which is usually 6 weeks.
To test before then you have to use a different test not a blood test
which some vets don't think is as accurate.

While its true that a cat can test positive and later negative because
they have been exposed to the virus and fought it off, it is also true
they can test negative and then turn positive if the test is done to
quickly after exposure. All this makes it very confusing but its an
extremely small percentage that will come down with felv after a
negative test.

On testing kittens and having experamented with tests I have never yet
found a kitten less then 12 weeks to be positive where the kittens
mother has been negative and I've also never seen a positive mother who
didn't also have positive kittens. It might be a bit of a waste of
money having to test all the kittens and put them through that where its
possible to test the mother.