Could my son get cancer if he was diagnosed with pleomorphic adenoma, 3 tumors in the neck and 1 in cheek?

Rarely: Pleiomorphic adenomas are benign tumors of saliva glands. It is unusual for them to be in the neck itself, but they can arise "rests" of salivary tissue in the neck. They tend not to spread to other parts of the body but can grow back in the areas where they arose in about 20% of patients who had surgery. Rarely they can turn cancerous and spread to other organs. Best wishes.