I don't find it odd at all the William could get an A in math. When our adopted daughter was in grade school she got As in spelling---and she can't read or write worth a darn.
I'm sure you're relieved that you finally have a diagnosis for your son. Now, my advice to you is this: don't let the schools give you the run-around. Our daughter was tested every 2 years from the time she was 5. Each time we were told she could do mainstream work if she just applied herself. By the time she was in 7th grade she had fallen so far behind in the parochial school she was attending that we had no choice but to send her to public school where she would get the extra help she needed. We met with her teachers and counselors several times a year; at each meeting we were told what a cheerful girl she was, she manages her time so well, the teachers loved her---she was so helpful, she got her work done on time---and if it wasn't right, why, she could do it over and over and over until it was. She did extra work to bring up her failing grades.
At each meeting, I would tell them "she doesn't manage her time well--I have to hound her to finish the night before an assignment is due". I would ask them "if you let her do it over half a dozen times before she gets it right, what is she actually learning?" The graded on a huge curve---she could get a score of 55 (out of 100) and still receive a passing grade. She wasn't being graded on her school work, she was being graded on her personality.
Sorry, I went off on a tear, didn't I? It's just that the school system was pushing her thru, whether she knew anything of not. To this day, she can't write a paragraph that makes sense. She can't do any kind of math without a calculator, she can't spell, her reading comprehension is abyssmal, but that's ok because, according to the school, at least she tried.

(no, she didn't; she knew she would be passed thru without earning it)
What it's taking me a long time to get to is this: if William can do the work, don't let the school system push him thru because "he's such a happy boy".
I'm glad you have the answers, even if they might not have been the answers you wanted to hear. At least now you can take it from here and make sure William gets every bit of help he may need.