C'mon Uncle Ernie, actual letters? When you have video-conference, would you spend your time writing?
He can also forget the romantic side of a summer romance, have wild sex with her, and get on with his life.
One of the good things of summer romances is that if they are not genuine, they won't stand the test of distance. I have known many people who is burned with the flames of a new romance, just to fade away quickly so they just get on with their lives, hundred of miles away. It is easier when you don't see each other every day.
Personally, I date my wife for 8 years before we got married, and we saw each other one weekend every month. Maybe that was the key for our relationship lasting, but the fact is that had any of us felt the fire was gone, it would have been easier to split.
It all depends in which kind of persons both of you are. I would say go for it, and if any of you feel in the future that it was a wrong move, just be kind with the other and end up like friends again -specially if your mom's are friends-, at least you will have good memories. And hey!, you don't always have the chance to date a 7-years-younger-than-you girl!!!!! When I was 16, dating a 19 year-old girl was a blast, I was a kid and she was a woman. Now I wouldn't mind she was 7 years younger than me HAHAHAHAHAHAHAHA