I was the same way. I tried to persuade him otherwise. Luckily, I saved right before I talked to Riordan to chose my final party and I went back and took him out. Which meant I had to fight that blasted Archdemon again >.<
Now if I tell Morrigan no, I leave him at the gates
-goes and cries-
Now if I tell Morrigan no, I leave him at the gates