The question of your ancestor's literacy may not be as simple as he was or wasn't. There is the ability to write a name, speak the language, read the language, and write in the language. And even within those abilities, there are ranges of abilities.
And your ancestor may have been "literate" in one language, but not in the language of the country to where he immigrated.
Some aspects of our ancestors' lives can't be answered with a simple yes or no.