M.A.
Your child needs to know that parents need adult time. This trip will be so good for his independence. When he sees the two of you spending time together this will help him with his relationship with his wife when he grows up. Children need to feel cherished but they do not need to think that the world revolves around them. You are not only strengthening your marriage but helping shape his future values. If you get the chance start doing this at least once a year if not more. The trips don't always have to be 4 days, they could sometimes just be one night away just for Mom & Dad. As far as rewards just bring him a souvenier back from your trip (for good behavior at his cousins). I wouldn't give him anything before you leave because there is no behavior to reward yet. He will be fine without you. I know as a parent that we probably miss our kids more than they miss us. Do this (without guilt) knowing that you are doing a good thing! Enjoy your time together, you deserve it!
God Bless & safe travels!