I would tell him good-bye -- much like you do when dealing with separation anxiety at home -- or not actually good-bye, but something like, "Mommy needs to go get some water (or something like that), Mommy will be right back" and then disappear for a very short period of time and come back. If he makes it through that ok, then work on lengthening the amount of time you're gone. I would talk about it beforehand too, at 13 months he's understanding a whole lot, even if he isn't saying much at this point.
