What is the ruling on a father demanding that his son pay back the full university tuition expenses that he voluntarily covered, knowing that the son did not obligate himself to that, nor did he promise to pay?

University tuition fees are not included in the obligatory maintenance. If the father does not pay them as a donation, he has the right to demand them back. If he insists on their return, pay them to him as an act of piety and kindness towards him.