So I read the trilogy in the early 2000s and have been thinking back on it. The main character is named Axel. He travels from where he lives with an old wise man to the main city. On the way he figures out that he can kill a dragon with a fire arrow by shooting down its throat right before it breathes fire. That's all I remember from the first book.
One of the others has him attacked by a barbarians who have invaded the country. I think they were covered in blue paint. They end up being friends and helping each other out. Axel drops rotten eggs on the barbarians to drive them away from the castle.