J.,
One thing you can do is play peek a boo. Start right in front of him, and gradually progress so that you are moving out and about. Move to different parts of the room, so that he looks for you, then pop out the door but keep your head in view. Then move back in and gradually move further and further from the doorway, always coming back. Do this a little bit each day. You know, it may do something to help, it may not.
Your son may be helped also by baby-wearing. My son loves to run around, and then there are times when Mommy's arms are IT. Whether I'm doing laundry or what, I'm IT. Daddy is very, very rarely IT, bc once Daddy is there, and I leave to head to the restroom, all of a sudden, I'm IT again. :)
This won't last very long. Try to give him long, solid hugs, the kind with your arm along (in line with) his spine, and cradle his head with your hand. My son loves these hugs and the ones where he nestles his head right under my chin (so that I'm practically looking at the ceiling sometimes! :)
Rock him and hug him and kiss him and tickle him, then firmly say, I will be right back. Then turn and go to the restroom and when you come back, make a point of saying: See, here I am! I love you! Here's a hug!
Good luck!
M.