That's what I suspected. We have a female russian that is very female looking. This one threw me for a loop.With some concaving, I would also say male. I have a male that has never flashed and then another that flashed at 3 years. They are both the same age, give or take a month or so. My male that has never flashed was slow to grow due to its care before I rescued him. Which I believe is making him slow to mature. However, he acts very male, looks male after a long time looking female, and does have the concaved plastron, just not as concaved yet as my other male.
I would be within the next year he, yours, will show more obvious signs.