Most Helpful Guy
As long as it's not more than 10 years difference, then it's fine really. As long as you guys aren't doing any sexual, you guys got nothing to worry about.
Most Helpful Girl
All you can do is present him with facts. The facts are, your parent is fine with it, legally the relationship is acceptable in most states, and plenty of people have a 4-year age gap in their relationships.
However it's up to him to decide whether he's fine with dating a high schooler or not. He may be worried that the age gap will create more problems than what it's worth. For example, if he goes out drinking with friends, you can't come. He also may be worried that his friends will tease him for being a college age guy with a high schooler.