Science Diet is not a very good quality food. You didn't say which food you had to switch to so I can't comment on that one.

Cats are obligate carnivores, meaning they get 100% of their nutrition from protein (meat, fish, poultry). Also, cats tend not to drink much water; in the wild, they get all the fluid they need from their prey. So moist / soft / wet food should be a regular part of the diet.

When you feed a high quality food, there are no hairballs. Hairballs are NOT normal, though most house cats have them, because of the poor food they eat.

Can you switch him to a high quality food, dry and canned both? Something grain free like Earthborn Holistics or Taste of the Wild, for dry; and Tiki Cats, Weruva or Fussie Cat.

You can learn a LOT about cat foods here: http://www.petfoodratings.net/cattable.html

If you get him on a good quality food, and add moist at least once a day, you will see improvement. And THEN you can sort out if he needs a vet visit or not.