My bf is super skinny and it’s kinda worrying to me

My bf has a BMI in the 16’s, but it’s not because he doesn’t eat. I believe he eats a fair amount, mostly junk and soda, but he doesn’t gain weight. He told me he lost 4 pounds recently and he’s already super underweight. I know he wants to gain weight, but he is too lazy to actually put the effort in. Is there something I can do to help him gain weight?