When Dad has to go away the next time, have him take one board with him. When he calls you can help your son set up his board and play a game with Dad over the phone. The first few times it is very helpful to have a speaker phone so you can hear what is going on. Soon your son will get the hang of it and be able to play Dad without your help. This kind of interaction helps your son feel like his father is involved in his life even though they aren't together.
Pictures are another great tool.
Go through your family photos and select several pictures of your son and his father, and the three of you. Have two copies made of each picture. As a family project, put together two identical photo albums. When Dad has to leave, he can take one of the photo albums with him. During future phone calls he can turn to a page and ask your son to look at the same page.