My kids have ASD, but also ADHD, and they were different. The more ADHD kid rarely felt much pain, even when she was sick, so we did not notice much difference in her. The other whined more, but acted sick and slowed down too. I have known even typcial kids who did not act too sick even when they were.
How he feels and acts in many situations may have little to do with his nuerological status, as he ages you will find some things in his profile that are not attributable to neurological differences. When the diagnosis is new, you should make sure that you explore all the ways his neurology will effect him, just so that you know that you are handling things well, but it will become less and less at the top of your list for him as he gets older, which is a good thing!
Hope he feels better.
M.