My Son, Andrick, Who Is 6 Years Old, Chews On

My son, Andrick, who is 6 years old, chews on a chewy necklace for oral stimulation - he has trisomy 21. He drools and his mouth is always wet. This is causing him to have sores around his mouth. They look like acne (sometimes with a white head), and sometimes they resemble a fever blister/cold sore. What can I do to prevent these sores from coming or make them heal? I wash the chewy and his mouth often to try to keep the bacteria down. Thank you. (Rachel)