How can I get my in laws their green card?

I'm a US citizen and my husband is not because of his job. He has to travel a lot and going to renew his green card which is expiring by June. Recently, my mother-in-law passed away and I am planning to bring my father-in-law. I lost my parents long time ago and he is the only person whom I have to take care. I've researched everywhere and couldn't find any information about filing his green card.