I've always figured that it was a Zentradi/other Protoculture race for sure, and... personally, I go with a quarter, myself. The amount of time between SDF-1 and Frontier is just about right for two generations (and for the third generation to be 16-17), and as with Max and Millia we saw interbreeding going on pretty early on.

Headcanon is, that's why he knew Klan from such a young age, through his Zentradi side.