Hi M.,
I have a 2 1/2 year old boy and 6 month old girl, and we all pile in together at night too. We LOVE it - enjoy every minute of it (even the feet in the face in the middle of the night).
Could one of the grandparents possibly come over and do a nap with your son a couple of times before you go? Then they can just co sleep with him while you are gone and all will be well. Also, I would recommend making it out to be a big, wonderful, amazingly fun thing that he is going to get to sleep with Grandma, or Grandpa, or both, for the night. Talk about it a lot, and have the grandparents express excitement about it too.
Also, it might be good to have them sleep someplace other than the bed you all ususally sleep in. For example, maybe make up a Aerobed in the living room for all of them to sleep in, or in a guest bed - if they are in mommy and daddy's bed he might miss you more, but if they are in anothe bed it might be easier.
A night waking will be rough, there is no way to get around that, he will cry for you. But, he'll be with people who love him and he will go back to sleep...and if he doesn't, it will just be for one night an anyone can survive that.
Good luck and have fun on your night away!