Does the father sin if he sends his son to study in Turkey and the son falls into haram?

The father must raise his son well and keep him away from sins and temptations, as Allah says: "O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones." If he fails to do so, he is accountable before Allah, as the Prophet (peace and blessings be upon him) said: "Indeed, all of you are shepherds, and all of you are responsible for your flock."

As for sending a son to study at a mixed-gender university, it is not permissible except out of necessity or an urgent need. If the father predominantly believes that his son will fall into sins, then he is sinful because he has assisted him in sin, as Allah says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." However, if he has not neglected his son's upbringing and the studies do not involve temptations, then there is no sin upon him even if the son falls into sins, as Allah says: "And no soul earns [anything] except what is [chargeable] to it, and no bearer of burdens will bear the burden of another."

We advise the father to direct his son to study at a university that does not involve temptations. If he is compelled to send him to mixed-gender universities, he must advise his son to avoid temptations, choose righteous company, and seek Allah's help.