Hi, Shawn and Rumble! I assume you are talking about a GCC (Green Cheek Conure), right? BW is absolutely 100% correct. This poor little thing has been completely traumatized from living under terrible conditions for years so no wonder he is so skittish! What makes things sadder is that GCCs are one of the neediest birds when it comes to attention from humans so although the conditions would have been bad for any parrot (they are all highly social species that physically NEED closeness and the feeling of belonging), they were truly devastating for this little guy.
I would suggest you sit next to his cage (never staring straight but looking at him from the corner of your eye -staring is a predator behavior) for hours and hours, talking, whistling, singing, offering a treat every now and then (if he doesn't take it from your fingers, just leave it there for him to take when he feels like it because this is not a reward but a token of friendship you are giving him). Once you see him eagerly approaching the side of the cage closest to where you are, you can start opening the door but do not put your hand inside his cage! If his cage is not the kind that has a door that opens down and stays up (like a moat kind of thing), put a perch outside his cage close enough to the door so he can just step on it from it. Keep everything quiet, no TV, no radio, no loud noises, no sudden movements, no children, no pets, etc. The idea is to reassure him that you are NOT going to grab him (how have you been getting him back into his cage?), that you are NOT going to force anything on him and that you are NOT going to threaten or hurt him in any way.
Now, I don't want you to be discouraged but it's going to take a loooong time to undo the damage done to him so arm yourself of lots of patience and think in the long term because this is not going to be easy for him but, if you do everything right and your persevere, you will have a friend for life!