What could this be?

My child's father passed away this year the doctors say of lung cancer. But he was not a smoker, and wasn't around chronic smokers his entire life. The diagnosis seemed unreal. He was asthmatic when he was younger and had to be put on steroids for a period of time. In his family I know there was a history of Sarcoidosis, diabetes, migraines, and asthma.

His symptoms were:

Chronic coughing
Vomiting Blood
Shortness of breath
Cyst on his lungs (do not know if they were fluid filled or solid.)
Could not keep food down
Pain
Loss of intimacy
His body was swollen while he was in ICU
He had 2 biopsy's and nothing.. everything was negative.
Blood test and everything negative.. the doctors could not find out what was wrong exactly.

They said he had lung cancer, and started him on Chemo.
He had complications a few days after the chemo, and he passed

I'm concerned because it was unclear what the cause of death was, and it's important for me to know because we have a 1 year old together. Who has already had RSV and Bronchitis. I would like to know if it could have been something genetic.