English, rare. [See Alison (F).]
A correspondent [VB] has a male ancestor, born 1882 in Fraserburgh, Aberdeenshire, Scotland, whose birth name was Allison. This name and spelling were also recorded for him on further certificates and on his World War 1 records. He was known as "Alex" or "Alec".
Another correspondent [GL] reports a more recent example in Canada [born 1957]. While it is a middle name in this case, he is known as Al or Allison.
These are the only recorded examples we have been able to identify of the use of Allison, or any spelling variation, as a male forename.