Extremely concerned about my husband's weight

My husband was over weight most of his life. Nothing crazy. He was usually only overweight by 20 pounds. 40 pounds over was the most he ever was and that was from when he was eating a lot from depression. His mom kept pushing him to lose weight and talking about the keto diet. His parents always made fun of his weight because he was the "chubby" kid. So he finally gave in and went on the keto diet. That was over a year ago. My husband is 6'1. He was 213 when he started. He has weight himself and he's 119 right now. Very significant weight loss and he's underweight now and I'm really worried about him. He has almost no energy to play with the kids. He's sleeping a lot because he's low energy and he's so restrictive of his diet. He also doesn't look healthy at all. To me he looks sick and I'm not trying to be mean. I am not sure how to talk to him about his weight because I know weight has always been a sensitive subject.