The white patches which he got treated for sounds like pityriasis versicolor. This has nothing to do with the premature greying. The most common cause is genetic predisposition due to which different people start greying of hair at different ages and it progresses at an individually variable rate. Pre mature graying is sometimes associated with nutritional deficiencies. You can consult online and i can inform which tests are needed. We can correct those issues is needed and add on additional treatment as needed. Hope this was helpful .