May I ask you a question, please? Are the parents gently following the child's lead? If so, I am not sure there is a long-term problem. I would feel differently IF it seemed either parent was stepping out of the child's shadow & trying to lead, making one choice or another seem right or wrong.
It surely could be a phase, but being accepting and open now means that when other issues come up over the next few years, even a complete change of gender identity, the child will feel secure discussing it with family instead of hiding it like it is bad. We have friends whose child decided at eight he was a boy, but this summer, at 13 has decided to be referred to as it/they, created a gender-neutral name, and informed family they will "decide exactly later." Fortunately, parents, brothers, grandparents and cousins are very accepting.
My heart goes out to those who struggle with gender identity, but family can make it not so scary by being flexible and accepting, creating a safe landing zone for the questioning one.

I hope you can accept whatever he tries on over the next decade or so. If you are uncomfortable talking about it with your peers, get in touch with the the LGBTQIA support community at your church or a local school, where you can meet others with questions like yours.
Sue