Talk with him about it and work with your fears until he goes. My T use to go away for 6 weeks regularly and once was gone for 3 months. Think of ways to "mark" the time until he returns with a few rituals. Get/make 4 notebooks, one for each week and journal in them "special" for the time he's away, it might help you feel more connected to him.

If you have any rituals of your own for when you go to your session each week, keep them up (I use to have a long drive and would grab a handful of the large, individually-wrapped Life Saver candies to eat driving to and from -- I'd do that for that day of the week when my T was away still; grab a handful for the morning when I left for work, etc.). Routine helps children and other human beings like us adults :-)