Note: Save your game before even entering Scrimsley as you can miss Amos. You can accidentally mess up this sidequest and you will never see Amos again, and thus be unable to recruit him. When you get to Scrimsley, the townsfolk will talk about how cool he is. Head to Amos's house and talk with him to learn he was bitten on the rear by a monster. He'll want to talk to you tomorrow and asks you to stay at the inn; do so.
You will wake up to a rumbling in the middle of the night. The Inn-keeper will attempt to get you to stay in your room, but ignore him and continue to the rooftop of the inn. Go over the edge of the inn to see a dragon-like monster attacking. Defeat this monster. Once you do, someone will prevent you from delivering, saying that this monster is Amos when night-time comes around. The Inn-Keeper will tell you more in the morning. Speak with Inn-Keeper the next morning to learn that Amos does this every night and the townsfolk won't tell him becasue he'd leave. The inn-keeper and some of the townsfolk tell you to not tell Amos. This is VERY important! You CANNOT tell Amos the truth if you want him to join you later. Go to Amos's house again and talk to him. He will ask if there's something on his face. Say YES, then a prompt will come up asking if you want to tell Amos everything SAY NO. Go to a mountainous area to the north of Scrimsley and get the seeds of reason. When you return, depending on if you told Amos the truth or not, he will be in town or he will have disappeared. If you told him the truth, the townsfolk are angry and an old women will tell you there's no use looking for him and everyone makes mistakes and she hopes you've learned from it (The mistake being if you're a completionist like me you now have to start over if you want Amos). If you didn't tell Amos the truth, give him the seeds of reason and the rest is cake. He will then join you.