welcome to HealthcareMagic
can be due to malarial fever;viral fever
& even in meningitis
.If the vomiting has started after the medicines it can be due to medicines also.Your doctor has given antibiotic injection and suspension of Bactrim
so hopefully his infection will settle down & he should be better in next 24 hours.If not he will get investigated further & treat accordingly.I guess the doctor treating your child is a Paediatrician If he is then follow his advice he will do the best for your child.
Wish him a speedy recovery.