Angelina, it's hard to know what the issue might be without further information. Does your veterinarian think Tigger should be gaining weight? There are cats who appear very thin their entire lives. Our spokescat, the black one you see in some of the images here on the site, only weighs nine pounds and she's a very healthy nine-year-old.
It's important to note that many different cat breeds can have what we usually refer to as a "tabby" color pattern to their coats. Some of them are naturally skinnier looking than others because they're descendants of Abyssinians or other very thin breeds.
The World Small Animal Veterinary Association has published this chart to help in the evaluation of a cat's weight. It's based on appearance as opposed to measured weights because cats can come in all shapes and sizes. No matter the size, you should be able to feel at least some fat covering Tigger's shoulder and rib bones.
Most cats will eat enough to maintain an adequate weight if the food they're given is high in quality. High quality cat foods are canned or raw diets with plenty of moisture, low carbohydrates, and with real meats as the main ingredients. Dry cat food is essentially junk food for cats and it should be avoided. See this post on cat food choices to determine if the food you've been offering Tigger has been good for him or not. If you decide to change Tigger's diet, you need to do so slowly over a period of seven to ten days to allow his digestive system to adjust.
If you and your veterinarian decide that Tigger really should be eating more, there vare several things you can try. Start by feeding him a food that's denser in calories, like kitten food. You can supplement this with some boiled chicken or other 100% meat that Tigger responds to. You can't feed him this all the time as it lacks certain nutritional elements that he needs, but it may be enticing enough to get him to eat more.
Don't feed him made-for-human fish products because many have extra ingredients that can be harmful to cats. Even canned tuna should be off limits, though it can be used in moderation to entice Tigger if you're desperate to get him to eat. A tiny bit that's been thoroughly washed and mixed into his food might do the trick. You have to be careful, though. Cats often develop allergies to fish proteins. In addition, some cats become addicted to the strong flavors in fishy foods and will stop eating anything else because other flavors pale in comparison.
Angelina, as in all things health-related, you need to develop a plan with the help of your veterinarian. Most veterinarians will have seen many, many cats over the course of their careers and they'll be in a better position to judge whether or not Tigger is underweight. If he is, then they should help you develop a workable plan. If they don't (or won't), it's time to visit a new vet.
We wish you and Tigger the best of luck in sorting this out!