Try it out with a keyboard first. If after playing 2 years the child still has interest, then shop for a piano - if that's what he would choose. Piano is not the only musical instrument.
Young children might like group lessons better than private lessons. My boy was bored by private lessons. The teacher was good. But there were no other children.
Music for Young Children is a good choice for young beginners. It is fun. It is more than just the instrument. Ask other parents to learn the reputation of the teacher. Also can ask the teacher for permission to sample one class. I took my boy to sampled the classes of 3 different teachers. All of them are good. My boy has preference with one teacher that he is most comfortable with. So we registered for that teacher's class