Did your dentist take an xray? It's imperative that he did this. Often, even though we take excellent care of our teeth we all seem to have places we miss when brushing, or "how" we brush may force food down below the gum line creating a pocket of bacteria, which then forms plaque. I find it difficult to believe that you would have periodontal disease this bad and not be able to see red,swollen and bleeding bums that are pulling away from the teeth. I would get a second opinion ASAP, I don't think your current dentist is on the right track with this. Take care.
Seeing a periodontist is advised.