How to tell my bf he needs to lose weight

So my bf is on the heavyset side and I do tell him about maybe going to the gym with me and eating healthier. Most of the time he says “okay” and it bothers me that he doesn’t take it seriously. My family loves him to death, but tonight after dinner my mom had a sit down. She said what an amazing guy he is, but she did say from the last time she saw him he did look a bit bigger. I told her how I’ve tried but she said I need to explain to him that he is putting his life at risk and i do understand. I need to try to motivate him more, to but I just need alittle advice on how to get him motivated and determined to lose weight. I fear so much that he could be hurting his health, and I want to be with him for as long as I live and the thought of losing him scares the shit out of me.