How to fatten up a Maine Coon kitten?

The best advice I can give you is feeding special "kitten" food, because it's designed to have more calories than general "adult" cat food. An alternative is food formulated for "active cats".

Dry food tends to have more protein than wet food, so it's better for your kitten. You can soak it in some water to increase the water intake. Please take care to feed a cat food with a high meat content and low grain content because grain can cause severe and very expensive medical problems in the long run (read more here).

Tuna should only be fed seldom and on very special occasions, but not as a regular food. The amount of heavy metals it's contaminated with is not healthy and has a stronger effect on the small body of a cat. You can feed raw or cooked ground beef instead, but ground pork only if it's well heated. Be aware that ground pork is actually hard to digest for cats and doesn't help fattening it up.

Another alternative that was recommended to friends by a vet is instant mashed potatoes. You must scan the ingredients, though! There must not be any salt, onion or garlick in it, because all of those are extremely unhealthy for cats. It's best to buy the pure, dried potato flakes and mix one tablespoon of dried flakes with some boiling water for a cat-sized helping. Feed this in addition to your regular food because it's not sustainable as exclusive cat food.