Aussie with weird Illness

Just wandering if anyone has had a dog with a like illness. Frankie is a 6yrs old Aussie and was just fine until June he came down with bronchitis. Was treated and seemed fine.Had all his Vacc.& heartworm preventive shot also in June. The first of August he started vomiting yellow bile and lost 14lb. Had bloodwork done and he was put on Meds for 2 weeks. He seemed to bounce back and then had a relapse. Dr put him on another 2 week round.On the 2nd week of that we took him in to another Dr and had sonagram done and another blood work up.No one seems to know what is wrong with him and said mabe take him to an internist.We have spent over $1000.00 on him and no one seems to find a reason.He is stll a very good farm dog and we love him dearly. I just dont know if we can keep spending on medical bills for him if there is nothing to be done anyway.We have 5 dogs and he is the youngest and we will try and do all we can.Its just sad to see him when he feels bad, but then he has times when he is his fiesty self.I just dont know what to do anymore. Im open to any suggestions at this point. Thank You,Suzi