Is a father's praise for his son after being told that he has memorized a Surah from the Qur’an considered Riya’ (showing off)? And what is the definition of Riya’?

Memorizing the Quran is not considered showing off (riya') unless your motivation behind memorizing it is to seek your father's praise. Riya' is when a servant performs a deed for people to see him and praise him. As for people praising someone without that person intending it, there is no harm in it; rather, it might be an immediate good tiding for the believer and an indication of Allah's pleasure and love for him, making him beloved to creation.