
You've done really well. First you say he's 3 weeks old, then 12 days, which is it? If it is 3 weeks then you should definitely start weaning now. Either get some pet mince (frozen to thaw out and give raw) from a petshop, or a good quality puppy food. If dry soak well in warm water and mush it up properly with a fork. You can add Welpi to it. I bet you he'll start eating pretty much as soon as you put it in front of him. If not, put a tiny amount on your finger and let him lick it so he gets the idea. Start with once a day then build up to 4 or 5 times a day. I have a Papillon (slightly smaller than your pup, he was around 100 g at birth) who was rejected by his mother at birth and I hand reared him. Mickey will be 4 years old this month and is healthy, happy, beautiful and still sees me as his mum. A spoilt brat really lol.
You also need to start worming now.
No, good breeders would never just leave a pup to die, if there was something wrong with it we'd have the vet put it to sleep. And no most vets don't know much if anything about rearing pups.
You really have done remarkably well, hand rearing isn't easy!