Congrats to your nephew! Based on your question, I’m guessing that you’re based in Europe because American driver’s licenses don’t use the same categories. A D1 category on your driving license means that you can drive vehicles containing 9 to 16 seats, like a minibus.
If you passed your practical car (category B vehicle) test before 1997 in the U.K., you will have automatically earned a D1 on your licence. You have acquired “grandfather rights” to additional driving privileges that your nephew doesn’t have, including the following category vehicles:
Drivers who passed their car test in 1997 or later, as in your nephew’s case, will only be entitled to drive category B vehicles. If they want to drive vehicles in other categories, such as a minibus that requires a category D1, they will have to take additional tests.