RNA - Abdullah al-Awda wrote in a post published on his Twitter page on Wednesday that his father, Sheikh Salman, had to fight criminal charges leveled against him without a lawyer, and that his family had not heard from him.
Saudi public prosecutors had earlier demanded the execution of the prominent Muslim scholar.
Awda is the assistant secretary-general of the International Union of Muslim Scholars, which the Riyadh regime has listed as a terrorist organization.
Saudi authorities detained Awda on September 7, 2017, and have held him in solitary confinement without charge or trial ever since. The officials have imposed travel bans on members of his family as well.
A family member told Human Rights Watch that the distinguished cleric was being held over his refusal to comply with an order by Saudi authorities to tweet a specific text to support the Saudi-led blockade of Qatar.
Awda, instead, posted a tweet, saying, “May God harmonize between their hearts for the good of their people,” - an apparent call for reconciliation between the Persian Gulf littoral states, the US-based rights group said in a statement.
Awda has been prevented from communicating with the outside world since October last year.
Saudi Arabia, the United Arab Emirates, Bahrain and Egypt all cut off diplomatic ties with Qatar on June 2017, after officially accusing it of “sponsoring terrorism.”
The administration of the Saudi-backed and former Yemeni president, Abd Rabbuh Mansur Hadi, Libya, the Maldives, Djibouti, Senegal and the Comoros later joined the camp in ending diplomatic ties with Doha. Jordan downgraded its diplomatic relations as well.
Qatar's Foreign Ministry later announced that the decision to cut diplomatic ties was unjustified and based on false claims and assumptions.
Saudi Arabia has lately stepped up politically-motivated arrests, prosecution and conviction of peaceful dissident writers and human rights campaigners.
Saudi officials have also intensified security measures in the Shia-populated and oil-rich Eastern Province.
Eastern Province has been the scene of peaceful demonstrations since February 2011. Protesters have been demanding reforms, freedom of expression, the release of political prisoners, and an end to economic and religious discrimination against the oil-rich region.
The protests have been met with a heavy-handed crackdown by the regime with security forces increasing security measures across the province.
Over the past years, Riyadh has also redefined its anti-terrorism laws to target activism.
In January 2016, Saudi authorities executed Shia cleric, Sheikh Nimr Baqir al-Nimr, who was an outspoken critic of the policies of the Riyadh regime. Nimr had been arrested in Qatif in 2012.