Hey.

If your brother took 1 Gap year and now wants to continue his degree he needs to get made a GAP year certificate. It is basically an affidavit stating that he did not take admission in any college or institute within this period and mention the reason for taking a GAP reason ( which will be to prepare for NDA exam in your case)

You would require a non Judicial stamp paper for this. Stamp paper for Rs 100 is ideal for this. You can visit a notary nearby who will take nominal charges to prepare sucha certificate for you.

Best of luck