Can you tell me how to tell my boyfriend i'm going to have breast reduction surgery?

Tell him the truth: You have a medical condition giving you significant problems. In order to treat this condition, you need surgery to reduce the amount of breast tissue. This will not affect your ability to be intimate. He needs to understand that your health is the most important thing. If you had a hernia, you would fix it. If you had diabetes, you would use insulin. Your condition is really no different.