Are you keeping a journal or diary of what is going on? If so, turn it into a one page document with graphics. Maybe copy it onto a calendar that has one page or type it into a format that has blocks or a spreadsheet so the doctor can see many a month or so at one time, not just 'my kid didn't eat today and hasn't pooped this week" thing.
In the meantime, cook foods he really likes and ignore him not eating as much as you can. Not for long, for a few days. You are going to have to take him into the doctor/ER quite a few times on this. Maybe take him every few days with that list all n one page but updated to be current when you take him. If he gets to the point where he is in serious pain, be sure to take him then. What happens if you refuse to leave until they do something? WOuld that work? You will have to take a book, drink and snacks (for you even if difficult child won't eat or drink) and a jacket if the waiting area/treatment rooms are cold. Take little for difficult child to do. Make him go find a doctor or staff member to bother - push him to tell them how much it hurts over and over if he is in pain. Just do NOT leave, esp if they get you into a treatment area.
The thing the docs are not listening to is that he is ignoring his body and is refusing to do these things. My daughter did miralax three tiems a day for several weeks and her obstruction did not clear. But hers had been building for months and most of her stool was eliminating past the blockage. the doctor didn't believe me and was going to send me home until J had a huge poop in the doctor's bathroom and I refused to let her leave until the doctor came in and saw it. No one could believe that that much could come out and she could still be blocked, but it was what had been happening for over a week at home. They did a second xray because the doctor thought the blockage would have been gone and it wasn't so we headed to the hospital for that milk and molasses enema.
I am sure the milk and molasses thing is NOT something down everywhere. It was a home remedy from an older doctor's mom and is used in our local hospital quite a lot for this problem. It CAN safely be done at home. But you would have to force your son and it owuld not work to do that.
Mostly you need to become a reporter on htis issue. Tell the docs what is going on, make sure that when difficult child is out of his mind in pain and refusing to go that the docs see him and leave it for a week or two. If you see a psychiatrist for the aspergers, call them and let them also know what is going on. They may be able to help or send you to a gastro doctor. Sooner or later something will give. either difficult child will start pooing and eating (either one will stimulate the other) or he is going to go for another two weeks and he will be so sick and weak that he will end up in the hospital on forced enemas and a feeding tube. NOT fun but you have done all you can but keep records.