Please don't let the fear of your doctor mentioning weight (as the cause of your pain) keep you from seeking treatment! To be honest, being overweight hasn't ever helped anyone's knees. I think we can all agree on that. But I would mention your weight loss and increased activity level. It doesn't matter what he thinks personally, but it does show that you are comitted to making yourself healthier.
I have had knee issues and have had two surgeries. My knee is bothering me again and I may have to go back to the doctor soon. Honestly, I look forward to telling him about my weight loss/exercise program. He suggested it over a year ago and I finally decided it was time.
Here's to good knee health!!