My DS had only one friend in K and "C" and he were constantly together. We met after school in parks, at the yoghurt shop, went to the museums etc together. She was equally gifted as him and she was the only other kid to whom he could communicate without modifying his thoughts or dumbing them down. They went to all summer camps together all summer long because she had problems fitting in with new crowds and DS was her buffer against all the noisy crowds and her helper who looked out for her.
We moved out of the school - she stayed back in PS - in this case, she was the one who had issues of missing her best friend, abandonment etc. We set up a playdate in October (a month after school re-opened) and went to the park and then dinner together. They played well together, but in that time, they had also grown apart. And interestingly, where in the past, they played exclusively with each other, in the new setting, they both made new friends of their same gender - my DS was finding other boys who were peers and she was getting more social and interacting with other girls in her school. The relationship wound down eventually when the extracurriculars became too intensive to accommodate playdates.
So, I think that you did the right thing in keeping touch with them - it is good for your son to not feel that the relationship got suddenly cut off. When your son finds new friendships, this one will gradually wind down.
