My 13 yr old male cat throwing up saliva and food after eating?

My 13 yr old male cat has been throwing up everytime after he eats, it usually starts with his head ****** and a raspy sound followed by a "blahhh" throw up. This started around 7 months ago, he was taken to the vet, who discovered a secondary issue (mass in his lung), after given barium test and xrays, Note: it took the barium almost all day to pass, when the Dr. said it should of taken only hours. He was put on steroids and wet food only and has been somewhat ok for a few months, but now again everyday for the past week, the vomiting is occuring everytime after eating. Back to the vet and a couple pounds lighter (down to 7 pds. now, he use to be 18 pd over a year ago). Back to the vet today, given a fluid admin. (I'm concerned of dehryation) and a cerenia shot. Since we got home from the vet five hours ago, he has vomited 5 times, he is hungry and can not keep anything down. Any feedback, suggestions to ask the vet, etc. would be truly appreciated.