Can a baby with after fever who has rashes all over the body to use fan or should he be using aircon in a room?

Unclear. Not sure i fully understand what you are asking, but if your baby had a fever and now has a rash, the type of air circulation in the room will not be likely to influence their recovery one way or the other.
Either is fine. The febrile infant should be appropriately dressed for their environment and kept comfortable. They should not be exposed to excessive cold (or heat) while sick. The skin has the only temperature sensors of our body & if the room is too cold the baby will likely push their temperature up, even if they have fever.