This could should not be fought on your own (being litigants in person), but via a law firm with a specialism in orthodontic and dental injuries.
Your Son being 16 has plenty of time to have his case prepared - it would not need to be issued at court until his 21st birthday.
I would strongly recommend you speak to a company such as Slater & Gordon Lawyers (they have a London office).
They will need independent evidence from another orthodontist but they can arrange this for your Son. The report would need to firstly establish causation and liability against your Son's treating orthodontist, and secondly to confirm the diagnosis and prognosis for injuries and recommendations for treatment.
They can claim damages for pain, suffering and loss of amenity, including an award for psychological injury, and any out of pocket expenses to include future dental/orthodontic treatment.
The firm can also offer a no win no fee arrangement - you would have to act on your Son's behalf until he is 18 years old so you would need to sign documents and so forth.
Slater & Gordon can be contracted on 0161(###) ###-####(head office).
I hope this helps - if you can please accept the answer and give me a 5 star rating (there should be a button on your screen to do this), I can answer follow up Q&A's at no extra charge and Just Answer will credit me for helping you today.