need idea what could have caused my kitten to lose use of ba

[Georgiesmomma]

I have had George since he was born to my 2 inside only cats. He was normal at birth no injury has occurred, he just gradually stopped being able to use his back legs. They actaully look like he has no hips, the limbs are not twisted but one sticks straight out. He can move them - and does when he watches his siblings play or when he wrestles with them- and he can feel them but he has no pain in his spine or hips it just seems like, he has no hips...any idea what this could be?Both of his parents are my indoor only cats that are disease free and his siblings are just fine....he didnt fall or get hurt he just got weaker in the back legs ...but one day when I surprised him when i picked him up he sort of freaked out and looked maybe like he had a seizure? But he is just fine otherwise, eats plays sleeps