He's obsessed with my weight

My boyfriend is an athlete. I'm not, but I do my best to stay active. I'm also naturally thin. However, that hasn't stopped him from commenting on my weight. He frequently tells me not to get fat and that if I did, he'd still love me, but wouldn't sleep with me anymore. I've never felt dissatisfied with my body before, but he's making me feel like I need to hit the gym a lot more. Idk what to do