High demands and inability to take "no" as an answer

Hi all.

I wanted to reach out to see if there's anyone else is experiencing something like what I'm about to describe. My son is 13 and about two years ago, he was diagnosed with Aspergers and potentially PDA traits. 

He's smart, he look people to the eye, he's got many friends but of course he's not good at reading social cues or understanding sarcasm.

Anyway, my question is about something that cause trouble and disruption in our lives. He usually have high demands, that he usually directs to his mother (we're separated). It could be clothes, expensive holidays, expensive gadgets, etc. He doesn't seem to be aware of the value of money and that people need to work hard to get it.

In line with this, he also comes up with eccentric plans, all of a sudden. For example buying land and building a house somewhere, hiring a team of architects and workers. A massive house with many rooms, and luxury. Of course all of that is absolutely impossible, but explaining that to him is pretty much impossible.

Every time that we need to say "no" to one of his constant demands (that 99% of the time require tons of money), the situation goes out of control. He will have a massive meltdown of destruction, violence, screaming and, something new, now he sometimes escapes the house and runs into the dark, during the night, on roads with plenty of traffic exposing himself and others to dangerous situations.

As his parents, we've learned how to deal with most of his challenges, but the inability of say "no" to him is, by far, the most difficult one because of course in many occasions that is the only answer possible.

So many times we end up lying to him, saying yes and hoping he will later forget or that we will be able to discuss his demand when he is calmer, but that usually triggers him again.

And is getting worse. For every new demand, if the answer is anything different to "yes", that will immediately trigger him, leading to high anxiety and meltdowns, destruction and even dangerous situation.

Please let me know if someone else is going through something like this, and even better if you have any suggestions on how to handle it.

Thanks in advance