I have concerns about a doctor telling a child of this age anything like this directly! A teenager is one thing. A 10 year old is something else, especially because this apparently never came up on a prior physical! I'd question the whole thing.
You don't do a recheck in 3 months. You go see a pediatric urologist or you go to an excellent children's hospital where they can can bring in urology, endocrinology or any other specialty needed, as well as psychological counseling/support.
You tell your child that you're going to check with another doctor and he shouldn't worry unless and until there's a real reason to do so. You are his parent and you will shoulder this burden while he goes on with the business of being a kid.