My 2 year old right now is going through a super-clingy stage, and it really helps him a lot to have pictures of us handy. Maybe you can put together an album for him to have at grandma and grandpa's of "familiar" things--his parents, pets, maybe his room, or the house?
That said, I sent my two (5 and 2) off w/ grandma and grandpa for a week this spring and was totally freaked out, too. But you know, they did great. They also do not see grandma and grandpa super-often, but they had a blast. They were out at the farm, my horrible-sleeping toddler slept through the night regularly, and they're excited to do it again this year. I'm still nervous. :) But...I think about how much we do for and how much we love our kids, and sometimes I think we do too much for them, and don't trust others enough to love our kids the way we do. It really is good for them to be with their grandparents, to build that relationship. It MIGHT be tough for him...but his grandparents love him very, very much (I'm sometimes surprised at how much my mom loves my boys; she was not very lovey when we were growing up, but now with her grandboys, she'd hang the moon for them) and will do their absolute best to make sure he is happy and well cared for. And he will come away with exciting memories.
Edit: Ok, I'm reading some notes people have said...ouch! I guess I'm of the opinion that yes, kids get upset, scared, and miss mom and dad--but that it's really, REALLY important for them to learn that you come back, and that YOU are entitled to YOUR life too. And just FYI...my kids have not had a baby-sitter in several years and they went FINE with grandma and grandpa--who they see maybe 3-4 times a year. We did call, but most of the time they were too busy to talk to us--I think we talked to them once during the week; otherwise, we heard about the day from grandma and grandpa. Grandma and grandpa brought the kids back to our house and spent the weekend with us, but I wouldn't even say our kids were particularly clingy after the event--and I'm a stay at home mom these days, so my kids were VERY used to seeing me, every day, all day.