Unless the nazi CCD nun is going to be the one giving your son Holy Eucharist, she won't be the one to have the say. If your Pastor or Assistant will be the one giving Communion, tell them what YOU want! If Extraordinary Ministers will be helping, talk to them before Mass. Or just have your son go to Communion with his hands firmly folded in prayer and his tongue sticking out. I doubt very seriously they'll refuse to give him Communion! When I'm helping give Communion, I have mostly people receiving in the hand, but Sir SuziQ always receives on the tongue.
This is YOUR son's First Communion, don't even discuss it with the nun anymore, go to the Pastor and tell HIM what you expect.
I decided to prepare our daughter for her First communion because I was disgusted with the quality of the teaching. When I told the Pastor what I planned to do, he asked me "Did you ask Sister Jane?" I said "No, I'm TELLING you what I'm going to do." He just laughed and said ok. But Sister Jane would not let my daughter make her Communion with the group during the special First Communion Mass; fair enough, she made it with us on Easter Sunday without all the silly songs, etc.!
My wife was furious. I thought it good example for them. My children have learned not to follow crowds mindlessly. They have learned to investigate the facts prior to action. They learned that priests they know and like sometimes do willfully engage in Liturgical anomie.
They are wonderful Christians and they have never taken the Eucharist in their hands as their Daddy endlessly repeated to them St. Thomas Aquinas' words "Only the hands of the Consecrated may touch the Sacred Species."
Lemme guess, Sister Jane looks like a guy and acts like a priest.