
Sorry to hear that your lab has HD and so young too, did he have exrays to confirm this? I'm surprised that this has been diagnosed at such an early age?
You should only be giving your pup short lead walks (about 10 minutes) for his age anyway so your vet is right about that, he will get all the play he needs running around in the house or your garden. Don't let him use the stairs or jump on and off furniture, all will prevent his HD getting any worse.
I would defintley get a second opinion ask your vet to refer you to a specialist but I would be quite careful about any surgery unless it is really necessary, dogs do not recover from this type of surgery as quickly or easily as people imagine, surgery should definitley be the last resort. If you take care of your puppy while he is growing, with limited exercise, prevent him from running up and down stairs etc there is no reason why your puppy will not grow up sound and healthy.
I had a dog with HD and we did see a couple of specialists, we were advised that no operation should be carried out on his limbs until he was fully grown and this was put at the age of 2. He was a retreiver so similar size to a lab.
I don't know anything about the operation you refer at 12 - 16 weeks but just going from what we were advised it doesn't seem quite right. Your dogs limbs and joints will be growing all the time and the operation would need to be carried out at intervals until he is fully grown which cannot be in the dogs best interest.
Please think long and hard before putting such a young puppy through such a traumatic operation, your vet seems wise not wanting to refer you for this procedure, maybe ask your vet to refer you to a specialist he has confidence in.