Has he been wormed recently,that can make them thin.
When you say -he will not gain his weight back- do you mean he had something wrong which made him lose weight, and now you are trying to build him up again? Or was he a rescue dog, with 'history'?
How old is he? And are there any health problems you are aware of? How does he behave generally? Does he have good energy, is happy and playful, eats well, does his business/pees ok? Has he been wormed, de-flea'd, had health checks at the vet?
It might be a good idea to take him for a health assessment, just to see if there are any underlying problems which might be the cause of this inability to gain weight. You could ask the vet to run a blood test, to check his thyroid, and see if there is anything else going on.
The only other possibility is if he is a massively active dog, burns far more calories than he consumes.
However, burger is a little fatty for a dog, and may contain onion which is TOXIC for dogs! So I would stop giving him that. Chicken and rice is ok, but is a fairly light diet. Perhaps you could start giving him a healthy-option dry dog food kibble, but not a 'light-diet' one, and add home-cooked (unseasoned) meat to this too. Don't give him fat though. Fat trimmings off meat are very likely to make him develop pancreatitis. The Omega oils a dog does need should be contained in the healthy option food. Fish also contains them, and is ok for dogs.