[troubletb]

I'm sorry to hear about your puppy, But if you didn't bring him in for his knees, what made the vet check for luxating patella?? Was it just a regular exam?? I know my vet checked my dogs knees during their regular exams too! Is your puppy having a problem walking? Limping? skipping a step? This is a #2:
Grade 2 luxations occur when there is occasional spontaneous lameness
but the patella returns to normal positioning easily enough that the dog
usually isn't pained much by it. This is typically the dog that
occasionally carries a rear leg for two or three steps on occasion but
then puts it back down and goes as if nothing was wrong.
I would get a second opinion on the Luxating Patelle, and go from there.
Good Luck let us know what you find out.

It sounds as though you are very concerned about you dogs weight, but as you were told before no one can guarantee weight..