Frist question, did you save him in the Unknown Dungeon way back near the start of the game? If you left him in the cage, he's gone for good.
If you saved him from the cage, than he should appear near Trader Pink in New Garand. He should also appear in your house(s) if you bought one. Some players miss seeing Adaon in New Garand simply because they forgot what he looks like and just over look him.