I'm sorry your little guy his having such a hard time. That must by difficult for all of you. Although your son's reaction is a little more extreme than I have seen among my kids and my neices/nephews here are a few suggestions we have tried with some success:
Don't make a big scene when you are leaving the house. Just give him a simple kiss good-bye and say "I love you". When you return home give him a big smile and a hug.
Play peek-a-boo games. Hide around the corner and then jump back into his view. Hide toys under blankets and help him find them.
When together in a room with him, gradually work your way across the room to put some distance between you. Go a little farther each day, even going into other rooms where you are still in his view. Work up to being out of sight. Praise him for being able to play by himself.
Good luck!