Hi H., good to hear back from you and that you managed to find such a special friend. I know what you're going through, as I moved to another country as a kid and had to leave all my friends behind. Most were too young and impatient to care to write letters and stay in touch as pen pals considering how slow the mail would arrive, and just moved on. These days though, there's incredible ways to keep in touch: texting, emailing, and my favorite, Skype. My daughter is able to keep in touch with family members and friends who live in other cities or states and still gets to see them via video and hear their voice.
When you are older, perhaps you and your friend can take turns visiting each other over the summer. Maybe even at this point, your parents would not mind sending you over to wherever his family lives during holidays or long weekends, if the plane fare is not too expensive. Try to view this as a future opportunity/excuse to travel, see new landscapes, and go away from home for even a few days. If he's not too far away, maybe once you get your driving licenses, you guys could meet up at a halfway point or take turns driving to visit each other, assuming the plane ride is not an option but your parents approve. There are also trains and buses that you both could take, if the drive is too far.
I did think that Wild Woman's idea of a memory book/scrapbook sounds like a great idea. It is thoughtful, will remind him of the special times and moments you spent together, and he can look back at the pictures of you two having a great time. In the meantime, rely on Skype, texting and emailing each other pictures and setting some time aside to talk on the phone. As others suggested, don't close yourself off to making other local friends too.