If he is that sensitive or distracted by sound, he may have a sensory processing disorder (SPD) and would really benefit from occupational therapies. Check out the book 'Sensational Kids' by Lucy Jane Miller and see if your kid matches any of the profiles, then try some of the techniques suggested and see if they make his life easier.
The noise reducing earphones only eliminate static (steady, unchanging) background sounds, like the road noise in a car, the whirrr inside of an airplane, or the constant hum of a window air conditioner. They can not reduce the chatter of other kids or a TV or a radio playing.
If he needs quiet but doesn't have to do any critical listening, home depot has a slim and springy plastic neckband type that has the earplugs built into the ends so you can't lose them. If being with loud sounds stresses him out,( like vacuum cleaners, lawn mowers, Harleys) these are great. They usually are bright swirly plastic safety colors but don't cover up your whole ear. Comfortable for short periods of time, they are very easy to put on or take off but everyone will know he's wearing them.
If he can't be that obvious about wearing ear protection, then get him some custom molded earplugs. A company called Sensaphonics in Chicago can mold his ears and make him a set for about $300. These are the same types that musicians use on loud stages and reduce sound evenly so that you hear all the detail just at lower sound level through the use of a replaceable filter that can reduce the sound by 9,15, or 25 decibels, and they are barely noticeable since they are molded to fit in his ears. They are not easy to put in as it requires a bit of practice to learn how to twist them just right when you are inserting them. Since they are molded to HIS ears, they are very comfortable to wear for a long time (unlike the squeeze and squish little foamie disposable earplugs that make your ears hurt after a while) and he will hear ALL the detail around him, so it's perfect for classroom use.
Good luck with your son, and congrats on looking to get him the help he needs.