13 year old Jack Russell diagnosed with mass under rib cage. How long does he have?

First the dog stopped eating had ultra sounds and then x ray all organs working fine. Kept throwing up bile and consuming large amounts of water. Vet gave meds he became disoriented kept feeding baby food, rice, chicken groundup. New vet says mass behind rib cage, new meds, sleeps alot, moves slowly so far keeping food down eats a little on his own. How long does he have? won't have surgery it would be to hard on him and he's 13. Member of our family what should we do for him to make him comfortable he's not in pain?