How to talk about weight loss with someone you love

My boyfriend and I have been together for a year and a half. I love him and I can tell we are close to the next step (marriage). Recently, I have become more concerned about his health. He disclosed to me that he weighs about 400 pounds. He was about the same weight when we started dating and it never bothered me. I have always struggled with my weight and I know how it feels to have someone discuss my weight with me so I never want to hurt him in anyway. However, I am concerned for his health. I am not sure how to broach the topic with him because I don’t want to hurt him. I have asked him to come to the gym with me because I want to lose weight and have someone motivate me because I struggle and I have discussed changing our diet but it does not really move past the conversation. What do you think? Am I being too judgmental? Should I broach it another way?