I asked my son to give an example for acid salt. He wrote aluminium hydrogen sulfate- al(oh)3+h2so4 ->al(hso4)3+h20 and he balanced the equation. But in textbook, aluminium hydrogen sulfate is not given in examples . Is this correct?

It does exist, Google it, there are many examples. Formula aluminum hydrogen sulfate: https://biturl.im/mCHYP