Fever since 20/05/24. Do blood test.

My son has had a running fever since Monday ie 20 May 24. We have consulted a doctor who earlier suspected viral fever but later as my son developed loose motions and high fever 103°F he gave an antibiotic Cefixime yesterday night. He has now developed severe loose motions after starting dose of Cefixime and his oral food intake has reduced to less than half of what he used to take when he was fit. I am giving him ORS and his motions have become less runny since morning but he is still getting fever thrice a day around 100°F Max spike 101°F. How long should I wait for the antibiotic to completely treat him so that the fever doesn't come back. The doctor told me if the fever spike is lesser than before then it's okay otherwise he will do blood test.