A big thank you to those who advised me a few weeks ago about how to manage my DS's PTSD and a boy with ASD, who's obsession was triggering my DS's episodes. After meeting with the school and the other boy's dad we had a plan in place to manage the situation.
The plan worked...for a while. Fortunately or unfortunately it wasn't entirely successful and there was another episode which resulted in my DS vomiting on the other child 
Now, you may have noted the fortunately? I'll explain. The vomiting episode resulted in the other child understanding that his questions were making my DS poorly, this in turn triggered an apology for making DS sick - something which has caused a lot of surprise! The child in question decided that he would not ask any more questions about adoption because being sick is nasty.
So, a rapid result...just not how any of us intended!
DS decided that this boy needed some friends so has made sure he's now a part of his friendship group, they even have a system for when this boy needs time out to re-group. A lot has changed and I think it has been difficult at times especially since this boy really didn't have any friends, but everything has settled down now. I glad it's worked out even if the path has been a little dubious.
DS's new friend was collected earlier after a 1 hour visit at our house (1st visit and earlier in the process than any of us were entirely comfortable with as it was a last minute arrangement to help his dad out, but there had been some prep done), he was becoming a little stressed in the last few minutes but no meltdown (and his dad called a few mines ago to let me know that he hadn't melted since being collected either).
The advice on my first thread, and some of the other threads I've read on here, has been invaluable and I suspect will continue to be so if this friendship continues.
and 
for everyone who knowingly or unknowingly helped us.