Terry, I think the problem you're dealing with here is that from difficult child's perspective, HIS embarrassment is far greater than an abstract, remote, "other person's right to know" or "other person's right to have possessions back". He IS balancing it all out, and for him, the balance comes down squarely in "I feel terrible when I'm embarrassed. I must do all I can to reduce this bad feeling to a tolerable level."
You could use this. Instead of working on returning the items (for now), work on this:
"We are stuck in a bad situation. You feel bad because you feel really embarrassed. But if you hadn't taken those things, you wouldn't feel embarrassed now. So what was it that persuaded you to take those things? It must have been really strong, to be able to overcome this end result of strong embarrassment; or this embarrassment now must have not been considered when you got the urge to take those things."
He needs to talk about this, to try to describe what compelled him to take the things in the first place. Try to work on getting his recollection of why he stole, connected with the embarrassment he now feels. Ramp it up if you can and try to again directly link the 'urge to steal' with the visualisation of how he would feel having to personally face the people he stole from in returning things.
Next step - role-play. Teell him this is role play, this is practice, this is necessary to help him be able to overcome these temptations in future. You need his cooperation for this. You are helping him, not punishing him. He needs to know this or it won't work.
Get him to practice trying to steal undies (or something else) from you, with you supervising. Now, as his hand reaches out, tell him to visualise how he feels having to face the owner and return what he is trying to steal NOW.
You may only need to do this once. Or he might need to practice it a couple of times. But if you can really make the visualisation vivid, once should be more than enough, believe it or not.
Next step - after the visualisation, you need to do something good with him, something that makes him feel really good about himself. This should not be related to him eating or drinking anything himself, or being given something material. He needs to get a positive kickback from something non-ingestion, non-material. Maybe he could go with you to feed someone else's dog, or run an errand for someone, or bake a batch of cookies to give to a neighbour. Basically, get him hooked into a random act of kindness so he can get the warm glow from someone thanking him for his kindness.
Or you could both sit and watch a video together, or you could play a computer game with him. But it needs to be as close after the visualisation as possible, so he can mentally contrast the feeling good, with the previous 'feeling bad' and make it easier to FEEL the right way to behave.
Next step - work with him on how to return the things with a minimum of fuss. If he wants you to do it for him then I would consider it (but only AFTER you've done the role-play). However, he needs to have a letter of apology he's written himself, to accompany you.
And he needs to know that if he ever does it again, you will make him return the items personally. And he has to keep this image of himself doing this in mind, if he is ever tempted to steal again.
Also important to let him know - Aspies are generally very bad at lying. They try it, but get caught. it's easier, it's safer, you feel better if you make a choice from this moment on, to never try to lie. And if you do something which is going to require you to lie to maintain the fiction or to cover something up - you will get caught, sonny Jim. So if you are ever tempted to steal and try to say to yourself, "I won't get caught - I will lie about it if I am," then he must remember to tell himself, "I am VERY likely to get caught out if I try to lie because I am bad at lying. I shouldn't attempt it."
Think of it as obvious and basic as training a puppy to go on the newspaper. You need to keep it as simple and direct as that, and use his own feelings, emotions and drives as the engine for the lesson.
And keep in mind always - this is not punishment. This is not punishment. This is a positive lesson. I am his helper. I am working to build him up to be a good, honest, decent person.
Good luck. Yes, this works.
Marg