Is he staying with a familiar sitter?  I would start by talking to the sitter about different activities to try out or maybe would take his mind of of yall not being there.  Having a sitter that actually takes the time to spend quality time makes a difference.  Always be honest with him and tell him you are going on date night with dad, and that you will return at a specific time, and for him to have fun, and you will see him after your date night is over.  Let him know that when he is on good behavior that both you and your husband will give a special day for the three of you to do something special together as a family.  Get a jar and fill it with his favorite things to do, so when he is on good behavior for your date night, or whatever you want to call it, then he get to take a pull out of the jar.   Try that.  Just reassure him when you are dropping him off that you will be back for him, and don't stay long enough for him to pout and put up a fight.  Having a good caregiver while you are away makes a world of difference.  Make sure that you sit down and visit with this caregiver whether it be a relative, or friend.  Sit down for coffee or tea and discuss a plan that might make him feel more comfortable.  Maybe you just need to find someone else to watch your son.  Sit down and ask your son what he likes about the sitter, and what he doesn't like about being there too.  Hope this was helpful.