My boxer dog keeps vomiting?

my 10 y old boxer keeps vomiting,it started 3 half mth ago. It started with a cough,then constant vomiting.I took him to the vet on numerous occasions,they said there was nothing wrong with him gave us antibiotics.They didnt work then a week after christmas his gums were blue and we took him to emergency vet,and they told us he was dying a (brain tumor) apparantly.So just pallative care,we are now in march.So i decided to take him to another vet,for a 2nd opinion,this new vet gave him steroids,anti sickness for his vomiting,and he was back to normal for about 3 weeks then the tablets stoped working.But this vet wanted to keep giving him,antisickness injection that wernt working.He was/is still getting thinner.So last week i took him to another vet,3rd opinion,and these have told me he is not dying,gave me more antisickness,i cant keep down him,electrolyte which he hates wont even sniff it and antisepin for his stomach acid,none of it working.I give up does ne1 no wat is wrongb with him