Gingivitis?

My new kitty is 3 years old. He came from the local shelter where he was being fed Purina Cat Chow (which he refuses). I have a feeling though that he has been fed moist food his whole life, because he gets really excited when I open cans. I know he needs dry food, but how can I get him to eat it? Also, on his medical records, it says the shelter vet found gingivitis. How do I treat this? Is it possible he is not eating because it is painful? I will be brushing his teeth regularly, etc. What else is good for teeth?

Getting off the wet food is the first thing you can do. It really isn't good for the teeth. I would try setting out a bowl of dry food all day and then gradually decrease the amount of wet food he gets. My first guess on the dry food is that he is just used to the wet, so why bother with the dry. If he stops grooming himself or starts loosing weight, that's a good sign his teeth are bothering him.
They are making several products these days. You can try getting him used to having his teeth brushed. There are also treats for dental care. They also make a gel/paste you can just rub on the gums. I would go to the local pet store and see what they have available.
As far as the cause. The one is yes, lack of dry food to knock the gunk off. It can also be caused by having upper respitory problems as a kitten. Kittens who get off to a bad start have a high rate of gingivitis. If it is really bad at this point, you can talk to the vet about doing a teeth cleaning and then maintain his teeth from then on.