What is difference between in a bus and on a bus?

The correct sentence formation would be :

"He got onto the bus."

AND NOT

"He got into the bus."

This is because when a person boards a bus , it is in a standing position i.e. he does not make a half squat position to get in.Therefore the correct term is onto.

When a person goes inside a car,or auto , then into is used , as the person gets in by making a half squat/half sitting position.