Well, short of drugging him to sleep or tying him to the bed...how can you really keep him off of it.
If you tell him to stay off of it, does he listen?
When you left the hospital, did they say "no walking" specifically?
If he's walking, it must not be causing too much pain for him to tolerate.
I would try to minimize his activity and keep trying to get in touch with the hospital or doctor's office--did they ever get back to you?
Here's some info on kneecaps regarding children: "Babies usually develop kneecaps around the fourth month in utero, so they're born with them. But these kneecaps are made of soft cartilage instead of hard bone. They don't harden, or ossify, into "real" kneecaps until the baby is about 3 years old."
Sooooo....maybe the lack of an actual bone kneecap explains his motility at 19 mos.?