Hi OP - your son doesn't sound like he is being supported much, is there a SEN coordinator or lead in the school; out to be from what the local authorities say. He/she should be able to arrange for some deaf awareness sessions,eg from the charities, NDCS, BDA or RNID, to help his classmates communicate with him - eg by catching his eye or tapping him on the arm to say hello rather than saying it from a distance.
I think at that age it is better to inform his classmates about his hearing condition rather than to leave it hidden thinking that because it's not severe it doesn't need to be obvious/visible - not suggesting people are trying to hide it but at the moment it is hidden.
The hearing aid will be great for him and yes, you can get colourful designs - make sure you have a spare although I know they are expensive (like glasses, if it gets broken or lost, that week or days without it is not fun). (I used to volunteer with a deaf children's group and had deaf adult friends then.) Once kids realise what the situation is and are told teh right thing to do they often very good about trying to include a child who is having difficulty - certainly at that age at least. The children generally like their hearing aids at this age!
A good friend has two children with hearing loss, her older is now in a mainstream (very sought after and competitive) primary school, and other children have never been a problem, once they were given the information. He's never got teased about the hearing aids. The main problem has been having to accept how much the mental toll of the hearing loss can take on the child's energy (and hence the parent's) which you are now both discovering. It takes them double the energy and concentration to focus on a lesson than their hearing counterparts, because of things being missed. And of course some things that come easy to us -eg saying "sh" vs "th" sounds, might need more work. Does he have regular speech therapy sessions? (a therapist who works with deaf children as opposed to a general one). Sorting out the seating arrangements at lessons, activities and assembly is also very necessary to maximize their view of the teacher/instructor.
Ponce- there is a wide spectrum of hearing loss, and sign language often doesn't really have much relevance to children/adults whose hearing loss is significant (ie mild to moderate) but not profound (very severe). Many siblings with deaf siblings get used to the adaptations and adjustments to make but at a young age don't understand/need to understand fully what it means - I suppose it's a little bit like a brother not fully knowing why it is different being a little sister (a girl)? But he understands not to push her hard or play rough.
The NDCS & BDA (British Deaf Association) should also be able to put you in touch with support groups or other hearing parents of children with hearing loss as they can give you more firsthand info than us, I suspect, of the challenges and problems you've both faced, and helpful solutions.
rebl - I suspect the reason why they can't hear so well in the car vs noisy class has to do with frequencies and maybe the Doppler effect of sounds bouncing off...but maybe someone with more physics/engineering knowledge than I have can explain this. Often it's just easier for them to tell you where they can hear better or worse in and learn to remember which, isn't it?