During future phone calls he can turn to a page and ask your son to look at the same page. Your husband can use the picture to remind his son of the wonderful times they've had together. This will help him believe that you will all have more wonderful times together in the future. It will also be very comforting to your son to know that he is looking at the same thing Daddy is looking at. This is a small, but very tangible way of staying in touch.
Videos are also powerful.
Try to get lots of close-ups. You can play this for your son, over and over as a reminder that things will be normal again. Begin long-distance rituals. A two minute goodnight call at exactly the same time each night can build a great deal of stability into the relationship between your son and his father. This time of separation is certainly a challenging one.