Weight gain

Hi all! This might seem strange as it’s not a health related question for me but for my boyfriend. He has always been quite thin well before I met him however he is underweight and very self conscious about it. He is less than 130 lbs and 6 ft tall. His clothes fall off of him and he wears multiple layers to make himself look bigger. I ALWAYS try and boost his confidence by telling him how sexy and handsome he is but I also try and encourage him to eat a more balanced diet (3 meals a day and snacks). He really only eats dinner and sometimes a small lunch. He works very long days that include 9 hours of driving so he eats a lot of fast food. I cook dinner for us when he is home and will cook breakfast every other day but he doesn’t always eat it when I do. We usually fend for ourselves for breakfast and lunch but I would like to start cooking more if it would help him. What can I do to help him gain weight. He won’t eat enough himself unless I cook for him.. he has no interest in the gym to build muscle. What can I make for him or add to meals that could help him gain a healthy amount of weight? I just want to help him honestly. Meal ideas? Supplements? I don’t know. Help a girl out!