Ahh, I remember that phase! I couldn't even go around the island countertop in my kitchen without my kids freaking out. I thought I would lose my mind :O
Here's what helped us:
-play peek-a-book
-play hide and seek (in obvious places for only a few seconds)
Make a game of not being there, then appearing. This builds confidence about your return. Eventually, do it using a door - letting him see you leave, then immediately coming back through. Lengthen the time you're out, and build on being positive and rewarding him with hugs when he's doing well with it.
Over time, you can make your away-time longer and longer, and viola, he'll be leaving for college - JK!
Another idea: put some new toys or objects in a pillow case. Switch them up every couple of days so it'll be fun and a surprise for him. This can be a really valuable way to help keep him occupied, and encourage him to self-soothe. Also, Baby Einstein and sign-language videos are wonderful for keeping their attention.
...this too shall pass. Hang in there :)
Best,
C.