There will always be old ladies who shake their heads. And really, a loudly-screaming child is probably worth a headshake, don't you think? But it's just the way it is, you don't have to take it personally. If you are within speaking distance, you might offer something like, "Yow, that hurts, doesn't it! Sorry, new phase, we're working on it!"
A big smile goes a long way, too, even if they don't have the presence of mind to smile back. It will send a much more positive message than you cringing, punishing your child, or glowering back. Keep your heart open, and your little boy will learn good things from you.
For the next year or so, one of your son's primary ways of learning how to be in the world will be through imitation, and he won't be very disciminating. As he becomes more verbal, you'll be able to reason with him more, so the mimicry will be less central (though it does continue to be important, so work on developing those reasoning skills before he's a teen!).
My daughter had an exceptionally shrill squeal which she was happy to employ for all sorts of reasons – the main one being, I think, because she could. Since it hurt my ears, I assumed it was just as hard on helpless bystanders, so I would quickly cover her mouth with a finger or two. No pain, no shaming, just sort of a "fun" response. Sometimes I would pat her lips to create a "wa-wa-wa" effect, which she enjoyed, and it would distract her from the shrieking.
I also kept my responses flexible. Sometimes I'd look her in the eye (or get close to her ear), and whisper, which would often throw her into a different mode. Sometimes I would make funny faces, maybe cringe and hold my ears (she thought this was funny, and instead of encouraging more squealing, I could actually distract her with some other focus once she started watching to see what funny thing I'd do next). Sometimes I would pat my own lips, and she would imitate that, at generally a lower volume than the shriek. A few times I would just jump, or instantly throw my arms up, or drop something. She would stop the noise, in surprise, I think.
Mainly, I paid close attention to what got her going. I know this will be harder for you when your attention is spread three ways, but to whatever degree you can do this, it will pay off. I got so I could usually anticipate when she was about to squeal, and do something that would distract her. This is a degree of "bonding" that you may not have patience for, but I felt it contributed to a happy experience in parenting for me, and my daughter was (and is, as an parent herself) a delightful, playful, and flexible person.
Good luck to you. A small investment in those cheap, disposable foam earplugs might help you through this stage.