I agree with the points being made in lots of the comments that OP needs to sort out a dentist that will take on her son, and arrange him some form of mental health support.
However, I think some of the comments about the op having her priorities wrong are a little bit harsh in tone.
Yes the OP does need to prioritise her son, but having read other posts, about her son and daughter, the OP is having a rough tine at the moment, with more going on that may be obvious in this post.
The child's dad isn't supportive and only helps when forced. As well as younger children, the OP has a teenage daughter that has recently had a baby, and was ill herself. It has been a lot to deal with.
I think she is genuinely trying her best, and is listening to advice from so many people, she doesn't know which way to turn. If I was in her situation I think I would need support myself.
OP has really tried to support her son in relation to the accident and school, despite many people on the other post just telling her he should be back in school near enough straight away.
There seemed to be progress from the school once they saw her son face to face, but that seems to have fallen away.
Having seen him, and knowing his issues with returning to school, I think school could have pointed OP in the direction of getting counselling in some way. I thought all secondary schools had details of that, even if they only told her to look on the school website for links.
I think OP should take a few deep breaths and make a plan so she can see things are moving forward.
First is to talk to her son, and try and find out how he feels, what he is thinking, what his preferred way forward is, so she has a better understanding of his needs and can use it when speaking to others to get help.
Go on the website for the local health board and see if there is anything about dentists taking on NHS patients. If not go on the websites, phone each dentist and ask, explaining the urgency.
It could even be worth an anonymous post on some local Facebook groups to ask if anyone knows dentists taking on. I have seen similar posts and people have replied with useful info.
If that fails explore other options. There may be some that stull do some nhs work, even if mainly private.
Speak to the GP about some mental health support. You could also look for info on the schools website, council website or local health board website. Contacting an organisation such as the local branch of mind could be useful too.
There should also be some information on the school website about educational welfare and children not able to attend school for medical reasons, which may be worth looking at.
@healingqueen Hoping that you and your son, can both get the support the need. You also need to look after yourself- you can support your son, daughter and look after the little ones if you don't.