This sounds like I heard days before here in Romania (on forums) about teaching a child to write in kindergartens or before going to school (if homeschooled). The school teachers were afraid that the children will learn to write in a wrong way. They've just seen few cases like that and now believe NO child should be taught to write before going to the 1st grade.
But there are manuals that we can follow so that our children learn to write properly! If I want and see that my child would love to write, I just help him.
There are always great teachers, either in piano, or school or whatever. But our child has her needs. We just can't stop them from learning something because we might teach them wrong - wrong in the meaning of those teachers, not in ours. Maybe it would be a good way to find out what is her method of teaching - what does "right" or "wrong" mean to her, and what is she afraid of if she told you that. In this way you may find how to keep your son's interest and make him happy and also do something useful for the future piano group lessons.
I want to teach him piano at home. What's wrong with the fact that my child, by the time he gets to school or formal piano lessons, will know how solfege, notes on the piano and even play by reading a piece ?
It means he's good for another level, not a beginner one. And though I may find a good teacher here, all I care about is if that person is able to treat my child as an individual, not as a part of a group that is or should be at the same level - That's the reason I'll always try to teach my children things at home, even if they go to school.
What are the teachers afraid of when they hear a child has learned about things she's taught later in classes ? That they will work hard to de-program a child and teach her something new as - in their opinion - she was taught in a "wrong" way ? That they will have nothing else to teach them ? That the child will misbehave, won't pay attention because she'll be bored ? That they will have to rethink their method of teaching because they also have good students loving to learn new things and share their knowledge?
Sorry, Kizudo, I'm a bit ... upset about this kind of situations...
Hope you find the best way for your child to be happy.
Andrea