It's possible your son is experiencing separation anxiety. Though this most commonly starts around 8 months, I would guess that some kids get there a little earlier. But he's about the age that he recognizes that he is separate from you, and becomes concerned, when he can't see you, that you have disappeared from his life. The anguish can go deep.
When my daughter began seeming distressed when I stepped out of the room, I began playing peek-a-boo games with her, hiding behind doors and furniture, and popping up just before she expressed worry. Then I started leaving the room very briefly ("Mommy will come right back"), and then popping back in quickly. I extended my absences gradually over several days, and my daughter clearly was more relaxed after I told her I'd be right back. I also set her in front of a wall mirror that let her see me in another room through the doorway, hoping that that would give her more information about where I might be when I wasn't with her. And I would hide other toys of hers, and we'd go find them together, so she'd realize they still existed.
All in all, I don't remember her anxiety phase lasting very long, and I think the games might have helped ease her mind. It's a normal phase, very common. Here's one good link: http://kidshealth.org/parent/emotions/feelings/sep_anxiet...#