I am a current FM user and devotee of them, and I have experience with a modern up to date system, which bear in mind some of the anti-FM posters (by no means all) don't have recent experiences only experiences from school days.
I've used FM for 21 years and it has certainly evolved over time!!
The balance of whether he can hear anything else other than the FM system's input is a question for the audiologist to fine tune using the hearing aid's programming software and the FM system software, e.g. in Phonak's case FM Successware. It is certainly possible to have a split FM program so he has direct access in the hearing aids to the primary sound source (say e.g. a teacher) but also has hearing aid mics active so that if there is an environmental sound, a comment from another child, etc. he accesses those too. The software helps to balance the two signals the way you (and later he) wants them so that it might be 50/50 or 60% the FM input and 40% environmental noise, etc. On some systems you can change this balance yourself for different situations depending if the surrounding noise is likely to be valuable to him or distracting and pointless. You can set to have an FM only option like what most posters are referring to where all you hear is the FM signal and all other sound is blocked and the hearing aid mics are off, and there is very little use for that except in a really loud environment.
You need to be sure he can understand what is coming down the FM and any input which is coming through the hearing aid mic and that he can distinguish them from each other, that no signal is too overbearing, and that is a fine balance difficult to manage with a small child, but possible with patience and experience. His thresholds with hearing aids look pretty good so you will be able to speak to him with simple commands down the FM to check if he's receiving a useful input.
Modern FMs also have a lot of features which are helpful to parents of young children like being able to monitor the system to ensure he's properly plugged in and switched on, automatic program switching so if anyone speaks into the FM it activates straight away at the hearing aids, deactivating buttons so he can't end up in the wrong listening program, etc. which removes a lot of the concerns about FM with young children.
You can read a lot of opposing literature on the subject, interested parties like Phonak have published a lot of research on the benefits of FM with young children, but of course they have a certain interest in positive results!
If you have an opportunity to try it and see without a massive financial investment then I'd go for it, but ensuring you have an experienced audiologist (including experience fitting FM) onside to try it with, it's certainly not "fit and go" and many with older experiences of FM will have been given fit and go systems with limited control which made them not at all useful. I wear my FM boots all the time and I use them to listen to everything, TV, computer, my daughter, conferences, college classes, etc. they are the most indispensible aspect of my whole overall hearing aid experience, but it didn't come easy.
The other thing is to make sure that people don't use the FM as a Get out of Jail Free card to no longer support his communication needs. There's a good chance he needs the FM
and visuals
and lipreading
and sign support, not instead of those things, so he should still try to sit close to the speaker with a good view in good lighting, etc. and people sometimes forget that. FM might support the sound signal in less ideal environments, for example if you did go to the farm you expect noisy animals, he needs time to listen to the animal noises but also would benefit from a clean signal for you to talk to him without that being talking over the animal noises. It also gives him more freedom to roam and still hear, like if there is a game involving instructions and moving around the room.
Used well it can be very good, used badly it can be very bad. You have to get it set well and then use it advisedly in situations where it is beneficial and know when to turn it off, which is often the difficult part outside of dhh programs. Mainstream schools get the FM and slap it on at all times, and that's where it all starts to go wrong, but that's not his environment so it may well be very useful.
