He might not be the only one, he may be the only one known
. Kale's parents kept him hidden in homeschool for a long time, there could be other humans in the world, Kale is the first live test in the general furry population. Not going particularly well, but he's only 17 (I think). Of course this depends on the rules of breeding between species and between humans and Furries. We know that closely related species can breed and produce viable offspring (Tyler, Kale's younger brother), but it's rare, and we haven't seen any hybrids out of close genetic relatives, what would, for example, Connor's parents' hypothetical biological son. Is Connor evidence that they can't have kids? If this is true, Human Kale probably couldn't have kids with anybody, except maybe primates (hey, I wonder why he is dating a lemur
Also, if humans can breed with anybody, whose genes are dominant? If Human genes are dominant over furry genes, this plan makes sense, if they're completely recessive to Furry genes, than it could work but it will take a bunch more breeding (and controlled breeding at that) to successfully reintroduce a human population to the world. Of those two possibilities, Furry dominance makes more sense simply because they managed to completely displace humanity. That said, it's far more likely that there is shared dominance of some form, meaning that potentially a kid between a human and a Furry could hold a mix of human and furry characteristics ( cat-ears?), diluting Kale's precious human genome, but also successfully beginning it's reintegration into the population.
EDIT (new idea): What supports the idea that Kale might not be the only one, just the only one known, is that there is a command structure above Max, Kale's father, and Hugo. Perhaps they're only a cell of operatives tasked to watch over Kale and deal with various issues as they arise. Why would they need a command structure outside of Furthia if Kale was the only instance of humanity, the entire operation could be run onsite in Furthia rather than from a distant location. If Kale isn't alone, it makes sense that they would have various cells of operatives watching over each human to ensure their safe reproduction and development. Kale's adoptive father has grown to want the boy to discover all this.