Most fans speculate it would be Timmy Turner, based on his human form in the Land of Tainted Souls:
The skin color is caucasian, as is Timmy's;
The hair color of both is brown;
Both have Blue eyes;
Certain character traits (laziness being one of them);
His Anti-Fairy Godparents are Anti-Cosmo and Anti-Wanda.
Many of the traits can't be contributed to Mandy (both hair and eye color don't match her own), so must belong to the father. We hope that the identity of his father will be revealed in the near future, most preferably in the comics.