Definitely sounds like SA.
Do the typical things in the day to reassure him, play peek-a-boo often. Have your dh watch him for you tell him goodbye and go get the mail or stroll outside for a spell then come right back assuring him when you return that you will always come back. Never leave without saying goodbye. If you go to a room other than where he is in the day, tell him what you are doing and talk to him from there.
Play peek-a-boo in his crib too.
As for night time, that is tough. I was given advice to place a chair by the crib. When your lo wakes, do the pu/pd thing like Liam's mama said and sit down in the chair. Wait for him to cry a real cry, repeat the cycle. The next night move the chair a little farther from the crib and so on until you are out the door.
As someone pointed out to me, don't sit in a chair you usually share with him, maybe bring one from another room so as not to tease him.
I am NO expert, but I have gotten some great advice on this site