First of all, welcome to the forums, and THANK YOU for taking in this sweet baby!! I'm so happy he has you to care for him and with his best interests at heart!

Don't forget to tell us his name and post some pics if you have some, cos we love baby photos 'round here!
In terms of handling, you may be in for a bit of a long haul. Cockatiels even at the best of times can be a bit flighty, particularly when we want them to do something they may not want to do like get back in their cage. Mine are hand tame but will run a mile if i need to cage them because i have to go out!
Are you able to move his cage into a smaller room before letting him out? Bearing in mind when he *does* get out, he may panic and fly into a wall or something like that, so perhaps having him in a smaller room where he can build up less speed before hitting anything might be a good idea, and then graduate to larger rooms when the panic stage has worn off a bit. I think you've done a GREAT job so far with getting him to trust you as much as he already does, because trust is often a long time coming with parrots, even more so when they've come from an abusive or neglected background like yours has. In the initial stages, I would also only let him out when you have lots of time to spare, thus lessening the possibility that you might have to catch him. He might just get back in his cage by himself after a bit of exploration, and being in a smaller room might help make it easier to catch him in the event that you may have to do so to get him back in.
Have you ever had a pet bird before? Apologies for giving you this article to read if you have, but if not there are all sorts of hazards around the house that you may not be aware of, so I've linked this up just to give you a bit of added info that might be valuable for you ...
Bird ownership is a challenge for the best of us, especially in the beginning. So many things are dangerous or even deadly to parrots. The information and hazards can be overwhelming at first. We've all been there, we've all made mistakes. Straight to the point. Open flames-any open flames...
www.parrotforums.com
When I say it may take some time - I once took in an escaped cockatiel from the streets MANY years ago now, I'd hoped she was hand tamed but turned out was most definitely not. She was a pretty little lutino whom I named Twinkle, and it took eight YEARS before she realised I was not going to eat her!! One day, after all that time, it was like a light switch flicked on in her head and she became just the sweetest cuddliest lovingest little birdie imaginable. So don't ever give up, take everything at his pace, even if it's glacial cos sometimes it is, and try to make every interaction a positive one. Your birdie is already 1000% better off with your love and care and that trust WILL come, hopefully in less than eight years for you!