Is the father considered sinful if he does not inquire about his son, who lives with his ex-wife in another country and whom he has not seen for years, and merely sends financial support, especially if the son is going through difficult circumstances and needs him?

His father has rights over his son beyond mere financial support, including good upbringing and discipline, and protecting him from forbidden things and what corrupts his morals. The noble verse: {O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones} shows that the father is responsible for educating and disciplining his children. This includes commanding them to pray at seven and striking them for it at ten, and separating them in their sleeping places. It was narrated from the Prophet, peace and blessings be upon him, that whoever has a child born to him should give him a good name, raise him well, and marry him off when he reaches maturity. Every guardian is responsible for his flock, and parents neglecting to educate their children corrupts them. The father must do what he can for his son, and supplicate for his righteousness and well-being, because a parent's supplication for his child is answered, and longing for the son is not a religious obligation.