My labrador is underweight I think, He is 2.7 Kg only at the age of 1 Month & 1/2 months old?

6 weeks is awfully young for a pup to already be separated from his litter and mother. How long have you had him? Can you see how other littermates are doing? Do you have any history on your puppy about his mother and father. Has he been wormed? Puppies get their immunity against viruses from the colostrum that is in the mothers milk when they nurse in their first 24 hours of life. This immunity starts wearing off around 6 weeks of age which is why it's so important to start their series of vaccinations as Erica recommended and he definitely needs to be examined like Erica and Walt recommended.

Anything you get online is not always trustworthy. The only person qualified to give out advice is a Veterinarian. Or a knowledgeable RVT like me (lol). Please spay or neuter your pup so as not to add to the horrible overpopulation problem.

The Royal Cannin feeding guide says you should be feeding your puppy at least 2 1/2 cup (211 g) per day https://www.royalcanin.com/products/r... I would also check with your Vet to be sure the puppy is healthy

I would recommend that you take your puppy in for a physical exam with your Vet. Your puppy should be going in for vaccinations every 3 to 4 weeks, so ideally he should be seen by the Vet who should address his weight.
Royal Canin is a great brand of food- I would follow the recommend feeding instructions that are on the bag. I would also recommend getting your puppy dewormed. (and of course this is in addition to having your puppy seen by the Vet.