I have had a fair bit of experience in the field in the last few months. Until recently, I had been doing no dental care; no brushing, special food or vet cleaning. Levi, who is about 6, has had gingivitis for a few years but I did nothing about it. The bacteria wound up eroding the enamel on three of his teeth causing cervical line lesions (irreversible). This necessitated the removal of three teeth because these are very painful. $600 later I had learned my lesson. So I started brushing my cats teeth every day. Two months later, my 2 year old cat, Cleo, got the same thing on one of her teeth. It must have started before I started brushing. Needless to say, now after another $500, I brush their teeth everyday, put Prov-Seal on their teeth once a week (protective wax) and take them in for regular checkups.
For the sake of your cat's health and your pocketbook, take him for the dental and start brushing everyday! Also consider the ProV-Seal and dental diet foods (I don't know how effective Science Dental is).
Sorry for the long post, but my inaction has cost me a lot of money!