Calendargirl
Just as a PS, my own children learned to drive at 17, back in 1991 and 1993.
They were each given £250 for their 17th birthdays, for lessons, and they both managed to pass first time, the money was enough for all lessons and test.
Of course, no theory test back then, and they knew that when the cash ran out, they would be paying the rest themselves out of their Saturday job wages.
Hope that spurred them on.
And yes, I realise I am talking about a long time ago.
I learnt in 1976 and it was £3 a lesson and £6 for the test! I was earning less than £1 an hour, so everything is relative.
As others have said, generally the younger you are, the quicker it is to learn to drive. Unless the OP's son has had previous lessons, he has left it late and will take more time to progress than if he were 17. Your son should talk to the instructor about why he has only been driving forward. If he is struggling more than expected with the basics, perhaps he may do better in an automatic; although the lessons will be more expensive.