This happened to me 40 years ago when I was 8 and I'm an only child. I was the one that moved. This was before computers and I don't know why, but I never got to talk on the phone with my friend after that. It was the typical waving from the back window while she stood in the middle of the street. It was awful.
I would just explain to him that these things happen and he can call and talk to him as much as he wants and when they are old enough to have their own phones and facebook pages, they can keep in touch that way. I know it isn't the same, but with Skype and technology now days it does make it easier.
My mom actually tricked me when we moved. Even tho I was waving and she was in the street, my mom had said we were coming back for one more load. We never did. That was pretty traumatic for me. However, many years later we somehow found each other on fb and are back in touch. We did send Christmas letters to each other every year for many many years as well and that's how we were able to keep in touch.
Of course your son will make other friends. I know how hard it is because my kids have been through it and it's hard for them to make and keep friends because of disabilities. Just be there for him and listen to him and try to help him through it. It will be hard.