If you told him about his curse, and he has run away from Scrimsley, then you have missed your chance to recruit him. You cannot find him after that. On no account tell him the truth, because he'll run away (you were heavily advised not to tell him, also).
What you needed to do is lie to him or simply not reveal the truth, before you go get the seeds of reason. Only after you get them, with solution in hand, should you tell Amos about it and give him the seeds. He will then join the party.
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