It is after effects of surgery so it is better to consult your vet. There are many reasons for vomiting of a dog. The first one is "Heat stroke". This happens due to water deficiency and high body temperature. You can give it "Naproxen" in such fever.
In surgery, he lost some blood so it is weak. Improve its diet. Give it soft food as its stomach will not support hard food after surgery.
In surgery, he lost some blood so it is weak. Improve its diet. Give it soft food as its stomach will not support hard food after surgery.