Because the 1901 census said he was aged 9, born Liverpool, that area seems to be more likely than the IOM to have his birth record. The Free BMD site has 7 William Halsalls registered in Lancs between 1891 and 1893, 4 of whom have second names. You might have to buy more than one cert before you find the correct William, or you could be lucky first try.
If you take 1892 as the most likely year, and if he never used a second name, there is only one:
Births Mar quarter 1892 Halsall William Prescot 8b 695
If you have already looked at that one, try one of the other 6.

Sue