Hi, i also am hard of hearing and i do wear the inthe ear aid's. I find sometimes hearing gets in the way, i hear wrong..or only parts, so i find it easier to have a visual on the instructor. I also lip read so that helps a bit. I don't think anyone needs an interpreter to study Aikido.
I have been studing for over 1 year now, and have never had any trouble, in fact i got new aid's and found it too noisy, LOL. all it takes is focus and desire. as we say in our Dojo..don't talk just Do. I think your nephew has the gift of silence, his other senses should assist, like mine have. sight, touch are more important, I think it would allow him to remain focased on the Aikido, and not watching someone try and sign the technics, which are sometimes called out in Japanease.. So comming from someone in the same boat, i personaly feel Hearing impared are more fortunate..we rely on sight and touch..makes for better Training.
Wish you well.