My 11 year old just diagnosed with moderate gum disease, NEED ADVICE

My 11 year old just had their checkup. He has moderate gum disease, ( always a reluctant brusher) no cavities. He had X rays and the bones are fine. The dentist scraped off alot of tartar and said in a couple of years his teeth will be loose if he doesn't take better care of his teeth. I need some tips to reverse this ( dentist said it isn't too late) I already have him brushing twice a day, rinsing, using Listerine and flossing ( gently) The doctor elaborated on proper brushing technique but I'd like to hear it again. Also , what toothpaste should he start using? Is pro health good? Thank you doctors!