You already got some really good advise on this. I was just going to put my two cents in and say that my oldest child loved to hear herself squeal for fun, but since I knew that my nerves wouldn't be able to handle it all the time, I tried immediately to correct it. I started out too negative with her, I feel. Eventually, I figured out she responded better if I told her happily and with a smile on my face the appropriate way to express that. Voice control games are a huge help. This made being quiet more fun. When I knew she "got it", the volume control thing, that is, I made a rule that we don't squeal in the car. That's too small an area, and I have sensitive ears. All I had to do was tell her "Remember, quietly (I whispered it to her) while in the car." Then I'd put my finger up to my lips and exaggerate the "shh". I think she quit squealing by the time she was 2. She still talks loudler and louder when she's excited, though, (just her personality) so even at age 12, we still have to work on it.
My second child screamed this high-pitch squeal when he was excited. Only when he was excited. He is my most compliant child, loves to please, so he wasn't trying to do anything wrong. Again, I had to approach it from the positive angle, and model some appropriate ways to show excitement. I'd smile really big and say, "Yeah" (quietly) and clap, or I'd dance side to side while pumping my arm in the air. The key for him not to feel like he was being corrected was to ask him to be quieter in a happy, positive way.
And a lot of these people are probably right that if you did nothing, it would quit on its own. Or if his ears need to be checked, well, hadn't thought of that one before reading that, but never a bad idea.
Have fun!