Well, to be fair to the shelter, just because he has FeLeuk doesn't mean that he wasn't tested. It's not uncommon for kittens to have false positives or false negatives, and many vets recommend retesting on 'negative' kittens.
If the shelter made a good faith effort at testing him, and provided you with all the information they had, you DID sign a contract saying they're not liable for medical damages.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry