20 month old with terrible plaque and staining on teeth! HELP!!?

Hello, my son is almost 20 mo. old. He has 12 teeth and all are terrible stained. They have brown moss like stripes and hardened plaque. I am very diligent is brushing (3-4 times per day) I always clean his mouth so that no juice or milk is left sitting. As far as juice he gets 1oz juice (cranberry or apple) to 7oz of water (filtered) he get 16 oz of whole milk per day and a polyvysol vitamin. He is on Zyrtec but I am not sure that can attribute to the staining. It looks terrible, I have to practically sit on him to brush. I am getting worried. I called the dentist and was told they could try to clean it but it would be hard because of his age. Has this happened to your kids? Any advice? PLEASE help!!