The AFCO no longer has any selection role, and does little beyond giving out leaflets and showing the pre-AIB video. If the applicant meets the basic educational and other criteria then they have to let them go forward to AIB.
The one thing your nephew must ask them for is places on the Potential Officers' course at Culdrose/Yeovilton. They should also be able to get him a ship visit for a few days. They'll give him plenty of bumph to read, and he needs to be strong on his knowledge of the RN and its role/assets when he gets to the AIB.