I have a lab puppy, he is 5 weeks old, the mother bit him and won't let him drink from her.
I have to feed the dog using a bottle, the problem is he is a quarter of the others size, what can I feed him to get him bigger, he only weighs approximately 1.1kg
At 5 weeks old this puppy doesn't need to be bottle fed, the mother has weaned him naturally (by refusing to let him nurse) because he's old enough, and needs to be, eating solid food.
I would choose a good quality puppy food, and feed him three times a day. Start by using the recommendations for amount on the food bag as a guideline, and give him as much as he can/will eat within about 15 minutes. He can drink regular water.
Although he's much smaller than his sibling in the photo, he looks healthy and not skinny. He is probably just smaller because his bones/frame is smaller and lighter. There can be huge variations in the size of puppies within a litter. Often it evens out a bit as they mature.
As long as your veterinarian has given him a clean bill of health, and he has no underlying illnesses or conditions, I wouldn't worry too much about his size.
Feed him well, give him lots of love and attention and I'm sure he will be just fine, even if he's little. Best of luck.