First, keep in mind that there is no "Italian" category - it's labelled Italy/Greece and it includes several other surrounding areas. What is your dad's known ancestry and what other categories does he have results in (and how much)? It's possible he has ancestry from a nearby region and it's just showing up under Italy/Greece instead because the groups are so similar. It doesn't mean he has actual Italian ancestry.
Secondly, be aware that the ethnicity percentages are just estimates. Note that if you expand the details, it will give you a percentage range, not just a hard number - what is the range for both of you for Italy/Greece?
That said, with your dad having 44% and you having 27%, it sounds most consistent with it coming from your father's parent, or your grandparent. As a comparison, I have one Italian grandparent and my Italy/Greece results on AncestryDNA are 31% (20-42% range). But it's impossible to say for sure since the ethnicity results aren't exact. If your concerned that one of your dad's known parents is not his biological parent, it would be best to look at his closest DNA matches - or better yet, have some of his known close family members test too (like a sibling or 1st cousins). The ethnicity results just aren't really a good indication of whether his known parents are his biological parents or not, or whether there was a non-paternity event somewhere else in his ancestry. This can only really be determined with his DNA matches. Or, if he suspects his known father is not his biological father, he might have some success with taking the yDNA test at FTDNA.