One thing that your son should do is speak to more people.
The groundschool part of the training is very unglamourous and can be mundane. Either way, it is still a requirement. If he has no motivation for retaking GCSEs etc. then passing groundschool subjects when difficulties are faced will also be tough.
Having a backup plan is important. I found during my degree, I learnt new skills. I also saw an improvement within my character. This would be beneficial.