A few things spring to mind that may be annoying him. Yes, there is a certain difference in sound quality - that’s usually a positive difference, but children often don’t like change even if it’s a good change, so it may just be that and he needs some time to get used to it. However, there are things it’s important to check on first.
Yes, Fm receivers can have distortion and interference - does anyone else in the building or nearby use FM? You may not necessarily know if there is someone in the surrounding area with something that interferes, but channel interference is literally painful! If he’s got interference then that can usually be fixed by choosing a new channel number. Do you have a hearing aid listening set? I am always astonished to find that parents have not been provided with one of these for children’s hearing aids. A listening set lets you safely listen in to the hearing aid at a lowered volume. You will then be able to understand what the hearing aid normally sounds like and recognise immediately if it sounds different. They need a listening set at the kindergarten also, so they can listen in to his FM and check it’s working and doesn’t have all manner of strange noises in it.
The volume of the FM needs to be expertly adjusted to balance with the hearing aid - who has done this? If nobody, then you need to find out who can do this work for you. If the Fm signal is overwhelming or incredibly loud then it’s not going to be comfortable for him. What is the program that is in use on the hearing aid? Is it an FM only or an FM and hearing aid microphone mixed signal? It shoudl be FM and mic together so he can hear environmental noises also.
I’m not quite sure what a junior kindergarten environment consists of, but I’m guessing it’s a lot like a nursery here in the UK, where some of the day is sitting down doing group tasks like reading books, but a great deal of the day is also dedicated to sand and water play, painting, building, moving around the room. It may be that the FM is being inappropriately used, he shouldn’t be connected to it at all times when he’s trying to play with the other kids, it’s for the times when the teacher’s voice is the main speaking voice and little else is going on. Sometimes inexperienced places get some equipment and strap it on 100% of the time because they don’t know any better. It’s very disorientating to use FM when you are moving around as you cannot tell where noises are coming from, they always arrive equally in each ear. It always sounds like the teacher has just sneaked up behind you and gone “boo” in your ears, can be very frightening. The signal is also broadly irrelevant during that time as the teacher is talking to another child, another teacher, doing something noisy that is nothing to do with him.
He should always know where the person is before they speak to him unless they are diverting him from an immediate danger. The FM should only be used to equalise with children with normal hearing, not to avoid having to move towards the deaf child and communicate with them properly, some teachers use it as a sort of a leash, constantly then talking to the child at a distance. They need to be using at as part of a set of tools for good communication, not as a substitute. He may also be having trouble talking to the other children if the adult signal is overwhelming him.
Other common FM problems include poor microphone positioning so the wrong sounds are picked up, teachers not muting the mic when they are doing something else (and they should use mute, not “off” if it’s only to be muted for a few minutes, as the on-off makes a noise in your ears). Equally remembering to put it back on if it’s been muted otherwise they assume he’s getting access to material he’s not actually getting. It’s also important that anyone wearing an FM transmitter doesn’t wear a load of jewellery like bangles that click and clack into the microphone, a necklace that bangs on it, a scarf which rubs on it, or that the person is a fidget and drums fingers on a table down the microphone, or shouts because they are looking for the attention of a far away child, forgetting that they are yelling into his ears.
Children’s FM fittings should have many of the functions disabled so a child cannot accidentally go into the wrong program, and that they don’t get a load of notification beeps in their ears all day long when the signal comes on and goes off.
Hopefully that gives you something to work with, is it a Phonak brand FM system? Do you know which transmitter it is used with? Some are more flexible than others and need different kinds of handling to avoid the common problems.