I think you should continue the contact and let him choose how much he wants to stay in touch, so long as it doesn't become significantly burdensome to you. If the relationship just naturally comes to an end because they grow apart, then at least he knows that you supported him. And maybe they will be lifelong friends (so much technology these days to facilitate staying in contact)
My son had a best friend in preschool, but when we moved schools we didn't make a big effort to sustain that bond. Now I wish that I had tried harder to have them spend time together (although, they have just moved out of state), because DS has been struggling to find that kind of friendship since. I feel that having an "anchor" friend is often important to help our kids build the confidence to create other friendships.
