Tips for gaining weight?

My boyfriend has had serious trouble trying to put on weight. He is about 5’10 - 5’11 and barely 115 pounds

He is extremely skinny, he looks very ill, you can actually see his heart beating through his chest if his heart is racing. His cheeks are starting to sink in. (He doesn’t do drugs)

He’s put on weight before, he actually got up to about 135 but then dropped it all rapidly, none of his clothes fit him anymore, they just fall right off. I’m really concerned for him; and he’s expressed he wants to gain weight, and it’s like he just can’t.

It seems to run in his family has his brother and sister are the same.

I’m constantly trying to lose weight, I’ve never thought of how to gain it. Help?