My bf is underweight and wants to gain it but doesn’t know how

My boyfriend (18m) is considered underweight but has been for a fair amount of his life. The only issue is that he has been losing weight lately and wants to gain more weight. He has a very fast metabolism and I’ve seen him eat like a garbage disposal. I’m just getting worried because he’s already very thin, ribs very prominent and almost sickly. He’s asked his mother for advice but she can barely speak English and just told him to eat more rice. Is there anything he can do to gain weight, or even just maintain what he’s at? tyia!!!!