What is the ideal weight for a 15 yr old boy that is 5'9?

Ideal actually varies from person to person. There are other factors involved such as the boy's frame, body mass index, and fitness level. A person that has 150 pounds of muscle will look much different than a person that has 150 pounds of fat. If you are concerned about your weight, you should talk to your doctor. They can tell you a good range for you personally.