Unfortunately there is no further application that can be made at this stage to extend the deadline. This deadline is one imposed by the court at the time of sentence and the course provider will not facilitate the course beyond that deadline. It is the responsibility of the individual wishing to take part in the course to ensure the deadline is met.
It may be worth contacting the court in writing to ask if they can vary the deadline, if there are extenuating circumstances for why the deadline was missed, but this would be at the court's discretion and is extremely unlikely.
Unfortunately the most likely outcome is that he will have to wait until the end of his ban. He can apply to the DVLA 56 days before the expiry of his ban to ensure his licence is reinstated in time for the end of his ban.