When you live in an apartment, you have to be considerate of your neighbors. It is not like having your own house. And kids need to learn the rules. No discipline is not kind to a kid - he will have rules all his life and he has to learn to get along with others.
But please don't punish a toddler for either happy sounds or for something he can't know he is doing wrong! There is a difference between stopping a bad behavior and showing a kid what to do and punishing him for a behavior that shows he is happy or that he is unaware of.
I was not very loud as a kid. But I do make noise when I am excited. My parents (hearing) told me about "inner happiness" - you can be wildly happy inside and not have others see or hear it. My (hearing) sister and I made up a sign for inner happiness when I was little - you just put both hands in front of your chest and jumble them all around with each other.
She calls it the "signing squeel". If I was loud as a kid, they would get my attention, smile at me, and sign "loud" with a funny grimace on their faces. At times they just gave me that look and didn't even have to sign it. But they always smiled and said they weren't mad or didn't understand. I'm sure I was more loud than I remember or know. I am glad they didn't punish me when I was making excited, happy noises or I would have become more self-conscious.
I also remember that when I was little (third grade?) I had HAs that hurt if I made a very loud sound when they were turned up to hear voices. Maybe Pete was right - his CIs need to be turned up. But I wouldn't turn the volume up without asking the audiologist.
I just reread the reply that said he got his CIs 2 months ago. I'm sure that seems like a long time to your neighbors, but he is still adjusting. Two months is not a long time for such a big change. Hopefully, he will learn with consistent patience work with you.
For ideas: Does your son go to speech therapy or see someone regularly about his CI? If so, you maybe want to talk to that person. They might have experience with loud kids and CIs and ideas for gentle lessons.
to you and your son.