I have not heard of it in one so young, but I do think it is worth obtaining a second opinion. I'm sure you paid a great deal of money for your darling puppy, and if you can find out now if this pup has a debilitating condition, it would be in everyone's best interest.
I have a Cavalier who was diagnosed with Syringomyelia/Chiari Malformation at age 4. I've spent thousands of dollars on his medical care, with terrific results. I understand though that you are between a rock and a hard place, because even if your puppy has this condition, I'm sure the pup is already a part of your family and owns your heart.
If you obtain a second opinion, and it is different from the one you've already received, perhaps you can work out something with the Breeder that is not just fair financially, but emotionally as well. A good Breeder wants to only breed good quality pups, and they want those pups in loving homes.
How familiar are you with your Vet? Have you used this person for a long time?
I truly wish you the best of luck. I do know about Luxating Patellas and you have some difficult decisions ahead of you. The best possible outcome is that your original Vet is wrong, and you find a new Vet.
Keep us posted.