From Alistair’s codex entry:
After Alistair reveals his parentage (on first arrival in Redcliffe if Alistair is in the party): Isolde’s suspicions were unfounded, however. Alistair was not Eamon’s son, but King Maric’s. Eamon sheltered the boy to hide his existence from Queen Rowan, Eamon’s sister.
If this is true, and not just what Alistair was led to believe, he cannot be Fiona’s child. Maric and Fiona did not meet until after Rowan’s death.
/runs away before head canon wars begin
So if that IS true, and not just a mistake between the book and the game (because there are differences) then who is Fiona’s son? Anders?
Doubt it, would be too easy. I’m thinking random unnamed second bastard. Hopefully one that can actually produce a non-OGB heir.