Is a father excused from visiting his disabled son in another country for fear of falling into the forbidden with his ex-wife, given that he continues to provide for them both?

Maintaining kinship ties (Silat al-Rahim) is obligatory, based on texts from the Quran and Sunnah. This can be achieved through financial support, service, visiting, and inquiring about their well-being. You are not obligated to visit your distant son; it is sufficient to maintain ties with him through financial support and inquiring about his condition, as long as his mother is looking after him. However, if you fear he might go astray, then it becomes obligatory for you to take him in. Visiting is not obligatory, especially if you fear falling into something prohibited.