As far as your son is concerned, there is nothing positive in terms of getting experience to salvage from this negative placement.
The most important thing is to explain that mean people there doesn’t mean he can’t be a great teacher somewhere else.
There is a painful realisation of reality here. Not all teachers are wonderful, and there are a few downright nasty ones.
Unfortunately as a young person doing work experience anywhere (not just in a school) you are at the mercy of your mentor. Although this was a particularly bad example, it is not uncommon to find a mentor who makes little effort with a mentee because they are not considered an important person who matters to them. It is not uncommon to find a mentor who makes little effort to support, inspire and educate but instead exploits help for menial tasks. Sadly, this extends to the different ways teaching assistants are deployed, so one lesson might be if you don’t want to risk being plonked with someone like that, train as a teacher not a TA.
Conversely, there are many many teachers who would give a completely different, positive experience of working with little ones. I would strongly encourage your son to be resilient and look for opportunities at other schools to volunteer as a helper, and to keep trying until he has had at least one positive experience.
There are many reasons which could legitimately put you off from teaching which can easily be googled, but the existence of selfish teachers like this is a minor one as they will be in a different room with a different class all day so you will only see them for 20 mins in the staff room.
On a different note, not that this will help your son at all, but I would write a polite letter setting out your son’s experience to his own head teacher and to the chair of governors at the awful school. Many head teachers would consider themselves far too busy to concern themselves with experiences such as your son’s, and I suspect this school has a head like that or he would not have had such an experience in the first place. Most governing bodies, on the other hand, have a measure of autonomy. They tend to be community minded volunteers who take interest in things like this and although it may be squashed and hushed up it may embarass the head into either providing a worthwhile experience, or at least removing their school from the arena to prevent harming more children like your son.