Alan should have been suspicious of his wife. Cory is the only one in his family who looks nothing like his family. They all have blonde hair. He has black curly hair lol ?

For all we know, Cory may have taken after a grandparent that was never seen on the show. And, actually, Ben Savage's RL mother doesn't look that different from Amy.

Yeah I noticed that Cory could have had a grandfather or relative with curly hair but from what I saw on the show Cory did not know people in his family with curly hair, which caused him to be insecure about it.