Howdy, you are obviously a very thoughtful parent and I think you have your answer already. So for what it's worth, I vote for some contact as well. Letters are good because he can write them anytime he is missing her and then the act of sending it is so essential to feeling heard and connected.
I met a best friend in 2nd grade and to this day we are still in contact, on Facebook. She's still one of the funniest people I know and we "get" eachother to this day, even with our completely different lives. I haven't seen her in person since a chance meeting in high school, but I always sorta carried that relationship with me. First best friends are very special.
