A have a 7 weeks old american puppie he is under weight. What would be the best way to get him to gain weight?

Has he been dewormed?

Baby formula. He should not be away from his mother yet.

At 7 weeks they should be on kibble, usually.

Ok, first question, when did you get this puppy?
A puppy should not be separated from its mother until it is at least 8 weeks old. It is illegal in most states!

You need to get your puppy vet checked as he will need to be checked and treated for worms.
You can buy worming treatments for puppies as young as 6 weeks but I would not recommend it without a vet check first.

With our pups we always give them just canned puppy food plus some dry food several times a day.

Those familiar with studies by behaviorists know that 7 weeks is a good time for puppies to go home. The practice is also well established in the real world.

Underweight? Not unless the ribs show. I have an extensive discussion of feeding in my sticky including a link to judging how much any dog should weigh.

Sariss is right about kibble. Only a dog food provides the carefully formulated complete and balanced diet puppies need. It is highly unusual for a healthy puppy not to eat all it needs.

Before everyone yells at me . This was a special situation. I got my dog at six weeks. She came from a litter of 14. She was fully weaned and mommy was too tired to care for all the pups.

the pup should be eating 3 X a day. With a good puppy kibble appropriate for the size of dog he will be i.e. large breed dogs do better with food that helps them grow slower.

the pup that came home a six weeks has been the most well behaved and social than any other dog that I have ever had. A pure joy and healthy too.

Had to spread the rep danielnoahmommy but yes, a dog separeted from its mother earlier may have no problems at all. No yelling here. I just think it's better to not take the risk if you have a choice.

A good breeder will know how to socialise a puppy properly as well not just throw it in a new home where the owners do not know about proper dog care and training.