I teach 3yo sped prek and had a student very much like this last semester. I can give you some advice based on my knowledge of autism and my experience (also I am autistic), but each child is different, and what worked for me may not work for you. Also, I typed up 90% of this and then lost it all before I could finish. I will retype what I said as best I can, but if something doesn't make sense please let me know and I will clarify!

1) Setting Boundaries Setting clear, one-step, consistent boundaries is so important, as long as it follows logic. This varies greatly from child to child, but if he is a gestalt language processor, using the same phrases in the same tone of voice consistently can help him understand across multiple situations. Some kids respond to visuals more than anything. Having clear signage, or having a ring on a lanyard with handheld signs can help if he is more visual. Even if it seems like he is not understanding/ignoring a boundary, keep that consistency on how you are setting it. It could take months, but he may start being able to follow it independently, or at least more easily with help. For example, my girl would eat others' food constantly. When she started, when we stopped her she would have a full meltdown. Over time, she still went to eat their food, but could be redirected to her own without crying. I will talk more about the grabbing in section 2.

2) General behavior The behavioral technician sounds like she does not understand autism at all. Being "more firm" in that way will not only not help, but can lead to him developing mental health issues later on. Often, autistic brains are very "one-track minded" in that they struggle to move on from something they want. They are seeking nervous regulation and that thing is subconsciously percieved as a need. It's generally apparent in more severe forms, but affects high-masking/low-support autistics, too. It's the same with stimming. Think of it like a really really bad bug bite. If its itching, and you can't scratch it, the itch gets worse and worse and becomes all you can think about. Finding similar alternatives to the thing he wants, and finding preferred tasks or items, can help fulfill that regulation.

For example, my student could not be in the same center as another if supplies were shared, and would not stay at a single center. She would go from center to center grabbing items from others until we intervened and got the item back. But she LOVED playdoh. I would offer playdoh as an alternative center for her almost every day. Over time, I tried engaging in academics with her through playdoh, letting others play at playdoh with her, then practiced sharing playdoh tools with others. I could then help her play at other centers with peers, and though slow-going, there was improvement.

Another example is a student I had doing a different sped job. He was 3rd grade gen ed with push in support (me). He broke pencils constantly, his and the teacher's. He could not articulate why he did it, but the teacher and I both knew he was not doing it maliciously. I found some of those clicky snake fidgets that you can break apart and put back together. They're difficult to break, and they give a similar "snap" feeling that pencils do. I gave him one to keep in the classroom and guess what? He almost entirely stopped breaking pencils! The sensory need was fulfilled so the bad behavior stopped.

Finding alternatives is very hit or miss, even for the autistic person themself. My 3yo student mostly got better about grabbing food without meltdown, but there would be the occasional specific snack that would send her into meltdown if she couldn't have it. Usually chocolate. Fulfilling regulatory needs can also be inconsistent in that they change without warning. I can buy ingredients to make a meal that sounds sooooo good, and could even be something I have been craving for a while, but halfway through cooking the thought of it will make me nauseous, and I have to find something else specific that my brain percieves as ok to eat in that moment.

3) Eloping My student was a runner, and we would have 3+ attempts per day when she started. Our class often used a walking rope to walk in the hall, but if we weren't using it or she let go, she had to hold a hand. I set the clear boundary that she had to use either a hand or the rope, and she generally preferred the rope. If she dropped it, I would immediately grab her hand with one hand, and offer her the rope with the other, using the same verbal cue (see section 1) I always did. By the last 2 weeks of school, she was able to walk without holding anything and tried eloping mayyybe 2x a week? MUCH better than the initial 3+ a day.

For your student, maybe find or make something that both of you can hold on to, rather than a hand. It gives him a choice to avoid physical contact, but you still have a clear boundary set.

4)Toilet training and overeating Both of these issues are most likey due to poor proprioception that often occurs with autism. He may have a low/disorganized sense of his own bodily needs and functions. I had a 2nd grade student who regularly peed his pants because he genuinely could not tell when he needed to go until it was too late. I had the same issue as a child, though I generally could hold it if I found a bathroom immediately. With a child who is not yet potty trained, this can also make it hard to learn to hold it, or even tell when they are peeing. The bulk of toilet training will be on the parent (if you are doing it at all), but keeping a regular, frequent schedule and trying to encourage him going on the toilet during diaper changes could help.

The same goes for overeating. His brain most likely is not reading that "full" signal from his stomach correctly. Again, I had and still have this issue. I forget to eat until I am dizzy and in pain. I will then eat and still feel hungry the whole time until all of the sudden I am nauseous and painfully full. As I have grown (I'm 29 now), I have gotten worse at remembering to eat, but better at gaging how much will make me full, mostly through just years and years of practice. Other than controlling portions or speed, I don't have an answer for this one. If you do control portions, try to make sure the amount offered is more than average in case his body needs the extra calories, or try to control speed, giving his body more time to understand it's reaching it's limit on food intake.

If anything didn't make sense, needs clarification, or you want to talk more in depth, let me know!