Pdi means he’s a trainee and has still to pass the final part of the qualification process. This is known as part 3, the ability to teach and has a national pass rate of approx 25%. That should give you an idea of how difficult it is to become a driving instructor. If his training has been up to scratch, then it shouldn’t matter. I started the same way so i wouldn’t criticise the use of PDIs, however, i have witnessed some poor trainees. Chances are, you will be paying full price for an hrs lesson, despite him being a trainee instructor and if you have a problem with this, then look elsewhere.