I have a sunnie too! He's 7. I got him when he was a little over a year old. I don't really have any helpful information about your question, just some thoughts.
For one, its not surprising (unfortunately) to hear of a sunnie being rehomed. Too often people are captivated by the colors and don't do their research. Sunnies are NOT a beginner bird nor are they for anyone who doesn't have enough time on their hands.
I do believe any bird can be tamed, its just that some take a lot longer than others. One piece of advice I can offer is this- if you can find out what things were done where he used to be you can have an idea of what not to do. For example, if his cage got hit with a broom regularly, he'll act up whenever he sees a broom and if you are using the broom- he'll then associate you with it.
Skittles will often freak out over 'new things'. I recently got a tablet and he wasn't happy- the lights and noises got him going. I 'introduced' him to the tablet and even put some treats on top of it to show him its okay and things are better now.
I can only offer this- patience cannot be stressed enough. They WILL pick up on frustration. On the plus side, sunnies are VERY social creatures who don't like to live in isolation, so you do have that going for you.
Maybe you can leave his cage door open during the day and wait and see if he'll come to you (ONLY if you are around to supervise and there are not any dangers (open windows, doors, other pets etc).
Try using treats. Chances are his previous owners probably locked him up alot with the cage covered. Skittles is free-flighted, but that's only because of my lifestyle. I make adjustments so that he has 'free flight' time daily. Since I'm usually home, he's usually out.