As a mom of five, you say you are used to the advice about how kids are communicating when they scream. But can you re-read your own original post and see why, before you SWH addition, so many of us figured you weren't getting that?
He is not doing it on purpose to "unnerve" and upset you, but please re-read your own post as if you were a stranger -- the post appears as if you almost believe he's doing it just to unnerve you, or because he might have Asperger's. I think your experience with your recently diagnosed child is coloring how you are seeing a very normal developmental phase here.
Distract, instantly. Don't ever yell in response -- that makes him think that the higher volume is acceptable. Let your voice drop lower. Do you talk to him a lot? Talking and talking to a baby often calms them, and helps them learn their words sooner, and distracts them as well. If he is particularly shrill, be sure he's in a safe location where he can't hurt himself and then leave the room for a few moments -- both for your ears' sake and to give him the surprise of your departure, which can sometimes surprise a kid into stopping for a moment. If he keeps yelliing, you need to first see what need he might have (diaper? food? some tag in his clothes poking him? a toy has fallen out of his reach?) and then respond to that need. If you have done that, then distract and/or depart. He may have to yell a while before he realizes that it won't get your attention.
If in the past, you have yelled back or shoved a dozen toys toward him hoping he'd take one and pipe down, etc., then he is learning that his loud volume gets mommy's attention -- that is GREAT to him, and he'll keep yelliing when he wants attention. Shushing him does not work because that gives him attention, and so does "No" -- to a child his age that is just a sound he hears a lot, not a word he fully understands. So if he yells, be sure to meet any immediate need he's expressing but if you have met the need, and/or tried distraction -- and it continues, and seems to be pure yelling for your attention, withdraw a bit -- but then give him tons and tons of attention as soon as he is calm and quiet. This is how he will learn that calm equals attention, loud equals no attention. But it takes time and patience, and you seem short on patience -- as any of us would be with a child newly diagnosed with Asperger's.
If you can get a break from caring for him, do it -- leave him with dad or another adult a few times a week at least, if you possibly can; you need a break.
Please do not ever throw water in his face as someone suggested. That will startle him out of yelling, sure, but also teaches him that mom's approach could mean an unpleasant startle any time. He might learn to be quieter but it will be out of uncertainty and a little fear of you, not because he learned to equate being quieter with getting more of your attention.