This was a fantasy book I was reading in the fifth grade in the mid 2000’s
Protagonist of the story was a young boy whose town was attack by a dragon. The boy joined a group of four or five other people to go hunt down the dragon before it would come back.
The group went through multiple obstacles to reach the dragons lair including a ghost in a swamp which took form of one of the character's friends who had died or had an injury.
Every challenge they faced the group got smaller until it was just the boy and the largest man in the group who lasted the longest because he was unflinching.
Eventually they have to crawl through a small compact tunnel to reach the dragons den. The large man started to break down because he could barely move in the tunnel and they crawled through it for something like three days.
Somewhere in the story the boy finds out that the dragon was attacking the town because he was hurt or had an issue so the boy helped the dragon with the problem and he didn’t attack the town again.