How do I help my BF lose weight??

My BF and I have been together more than 2 years and he's gained (and lost then gained) weight since we've been together. I'd say between 10-20 pounds. It doesn't make me any less attracted to him but I worry about his health as he has a history of heart disease in his family. Every time I try to bring it up he thinks I'm calling him fat. I don't know how to go about helping him and talking to him without making him feel bad about it.