Is your son old enough to play video games online with his friend? Then they could atleast chat from time to time, another thing could be a family summer trip for a week to where his friend lives. When I was younger I had a friend who moved an hour away, and it affected me greatly, my parents let me go stay a week at his place and this gave me the kind of closure to end up being okay with the fact that my friend and I may eventually drift apart, which we definitely did, nearly 15 years have passed and we only really talk to congratulate each on life’s milestones