Disagreeing on this part, and it's for this conversation during a semi-late briefing in Path of Radiance. Soren's bloodline is hinted at in this conversation as both Dheginsea and Ashnard are bought up and compared to what he says.
Maybe it's just me, but going off Soren's support with Ike... I had the impression that Ashnard had to of kept him around for a bit to see if any Goldoan abilities (if not outright shifting into a dragon) would manifest itself. When he discovered that Soren couldn't, it's not too much of a stretch that Ashnard just told a guard to take Soren away. This is after luring Rajaion to the castle and making him feral, for the record. Soren seems to have some faint memory ("I can hear him now.") of Ashnard. It's just not appearance or anything else he might deem important.
Ashnard wouldn't bother in remembering a child that ultimately failed (though he couldn't control his powers) in giving him what he wanted. Aside from Soren having a dragon's acute sense of directionality, he had no special abilities that Ashnard would call useful.