Thanks for taking in rescue!
Definitely use your other cockatiel to model behavior! Have her step up in front of him and give treats, then while she is in you have him step up for treats. I think safflower seeds make the best treat for many birds.
Break behavior down to little steps, you slowly shape to get the behavior you want.
You want the brain and body moving toward you, not you chasing after.
So for a step up, first you just say hi and out treat in treat dish, then walk away. When he always come to take the treat quickly. Then you move to only putting the treat in the dish if he looks your way, then if he means or steps towards the treat dish. Then he only gets treats if he comes all the way to the dish. Then when he always comes right to the dish, you start giving them by hand. (Even if he took treats before by hand do these steps, you want him moving to you to get the treat) when you are doing the above and are at the stage he moves to the treat dish , you can start saying come here at that time, you say come here when he is already moving toward you and the treat dish,and when he is coming to you to take the treat by hand. Then have him come to you , and take one step back to make him follow you to get treat. Work up to getting him to follow you for a few steps before getting the treat.
Now you are ready to work on step up. You have him come here, and hold one arm in front of him, and hold the treat so he has to step on the arm to get the treat. If you get to a block at that step,then brake it down again. At first he gets a treat if comes and stand right by your wrist or hand ( whichever you use for step up) then he gets a treat if he will mean over your arm, then we he does that well, he gets a treat if he at least touches your arm with beak or foot, tgrnbif he will out one foot in to lean over and get treat. Then he gets when he stands on you . The goal is to go at his speed, o my advanced as each part is mastered. Work in short pulses, maybe 3-5 times each session, do several session a day.
You can also try to shape letting you touch him. But I would do the above first. After you master step up. Then you can try offering treat by hand, and as he takes stroke his beak, wait till he us good at that, then hold on to the seed longer and try and stroke the side of the face.
Hope that helps!!!