Sorry OP but essentially he’s had a good outcome although you may not see it that way right now
Left untreated a torsion can kill- the testicle becomes starved of a blood supply and oxygen and dies/tots which can cause septicaemia and death.
So the fact the NHS promptly and safely operated has saved his life.
His presentation right up until the severe pain episode was atypical. Managing as possible infection (including screening for STD) and organising an ultrasound and then when all that was clear and pain still present, doing a urology referral was very good management by the GP.
Intermittent torsion is rare.
I suspect your son was told at least once by his GP that if the pain worsened significantly he should seek sos help- which he did.
Your son can discuss events with his GP but all a complaint will do is cause immense anxiety in the GP, take up time responding and involving the defence union which could be better used seeing patients.
All GPs reflect on practice all the time and when the letter from the hospital is received detailing the outcome of surgery, they will discuss and look at the case to see if any lessons could be learned
Believe me the vast majority of GPs hate feeling they didnt do all they could and are hugely bothered and distressed if they feel they fell short.
But in all honestly your son appears to have had thorough prompt and appropriate care and the outcome is that he is alive and well.
Sometimes people get ill and with the best will in the world it can’t be prevented.