I dont have a lot of advice, only that I'd not be comfortable leaving my dog with strangers. Is there any way to introduce him to the place and let him get comfortable with people that you know you can trust?
My pup is clingy too, they don't call them "velcro dogs" for nothing.
I'd be more concerned with taking him to a place I trust, and being sure he will be getting the proper care and enjoying his experience. Our training club does boarding too, and while I'd be worried about his confusion being placed with someone else while I was away, I trust the staff and know he'd get lots of fun attention and care.