He is the president of the Episcopal Commission for Pastoral Care for Migrants and Itinerant People (ECMI) of the Catholic Bishops' Conference of the Philippines.

[4] During his 27-year tenure as priest of the Archdiocese of Manila from September 10, 1983, to July 8, 2010, he became parochial vicar at the Immaculate Concepcion Parish in Pasig (1983–1986), chaplain of Pasig Catholic College (1983–1987), priest at San Antonio Abad Church in Maybunga, Pasig (1986–1987), professor of church history, patrology, and homiletics at San Carlos Graduate School of Theology and Holy Apostles Senior Seminary in Makati, visiting priest at the EDSA Shrine and Our Lady of the Pillar Parish in Pilar Village, Las Piñas (1990–1997), prefect of discipline of the Philosophy Department of San Carlos Seminary (1990–1992), professor of church history at San Carlos Seminary (1990–1995), academic dean of San Carlos Graduate School of Theology, Library Director of San Gabriel Reyes Memorial Library at San Carlos Graduate School of Theology, chief archivist of the Manila Archdiocesan Archives and curator and director of the Manila Archdiocesan Museum (1992–1997), guest lecturer for novices and postulants in the mother house of the Sisters of Saint Paul of Chartres in Antipolo (1992–1995), consultor of the Pontifical Commission for the Cultural Heritage of the Church in the Vatican (1995–2005), vice-rector and oeconomus of the Pontificio Collegio Filippino in Rome, Italy (1997–1999), rector of the same college (2000–2010), and national coordinator of the Italian Episcopal Conference for the pastoral care of Filipino migrants in Italy (2003).

He continued to serve as priest of the Archdiocese of Manila and rector of the Pontificio Collegio Filippino while being the diocese's bishop-elect from April 1 until he assumed office upon his installation as fourth bishop on July 8, 2010, ending the diocese's 8-month-and-4-days sede vacante period or without a bishop in office under the diocesan administrator Victor Ocampo, and his 4-month-and-7-day bishop-elect period.

Santos' installation ceremony or assumption into office as fourth bishop of Balanga on July 8, 2010, was presided by the then-Archbishop of San Fernando, Pampanga Paciano Aniceto.

Also present in the installation are Bishop of Malolos Jose Francisco Oliveros having jurisdiction over Valenzuela and Santos' home province Bulacan, Santos' predecessors as Bishop of Balanga Honesto Ongtioco (also the former priest of Archdiocese of San Fernando during Paciano's (installation ceremony's main presider) tenure from January 31, 1989, to June 18, 1998, and future sede vacante apostolic administrator of Malolos from May 11, 2018, to August 21, 2019) and Socrates Villegas, and outgoing sede vacante diocesan administrator of Balanga and future Bishop of Gumaca Victor Ocampo.

[3] Santos decreed the creation and addition of five parishes, four more diocesan shrines (only Orani Church was the only diocesan shrine in the Diocese of Balanga by the time he was appointed as the diocese's fourth bishop on April 1, 2010 and assumed office three months later on July 8), and one minor basilica, national shrine and chapel were created during his tenure as bishop of the diocese, with Residencia Sacerdotal, Retirement Home for Priests (Our Lady of Guadalupe Chapel) in Taglesville, Balanga started construction on late 2010 months after he assumed office as fourth bishop of Balanga on July 8 of that year and finished in 2011.

He made two quinquennial visit ad limina in Rome on December 2, 2010 (five months after he was installed or assumed office as fourth bishop of Balanga) and May 20, 2019.

Months before his tenure as Bishop of Balanga ended on July 22, 2023, the cathedral's leftmost door was removed and replaced with a wall.

He founded a foundation named Ad Seminandum RCS 10 Foundation, Inc., with the name being a reference to his motto as the diocese's bishop (Ad Seminandum), initials (RCS), and 10 being the month number of his birthday (October which is the 10th month of the year) and part of a number of a year when he became Bishop of Balanga (2010).

[7][8] In January 2023, the Diocese of Balanga mourned the passing of Pope Benedict XVI who appointed Santos as the diocese's fourth bishop on April 1, 2010, saying that amid sorrow and sadness, “we are assured of his eternal peace and rest in Heaven.” He said that the Bataan faithful were in grief over the death of the pope emeritus on Sunday morning at the Vatican at 95 on December 31, 2022.

On April 1, 2023, coinciding with Santos' 13th and final year as fourth Bishop of Balanga, the Diocesan Shrine and Parish of St. John Paul II in Roman Superhighway, Brgy.

Culis, Hermosa, Bataan was elevated into the national shrine on a mass presided by Bishop of Kalookan Pablo Virgilio David through the authority of Santos, and its second parish priest and first rector Anthony Sibug.

The mass for the repose of Dumaual's soul and his funeral, and blessing of the body were presided by Santos' predecessor Socrates Villegas who returned to Balanga, Bataan for the repose on July 25 and Lavarias for the blessing and funeral mass on July 23 (a day after Santos became Bishop of Antipolo) and 31, respectively.

He stated that there are conferences, seminars, and marches against the RH bill, with the goal of the “diocesan-wide campaign” is to protect the sanctity of human life and the family, he told CBCP News.

[13][14] Like with his predecessor Socrates Villegas, he expressed opposition on the revival of Bataan Nuclear Power Plant (BNPP) as it will put the lives on the brink of danger and shadow of impending death, result in the disruption of ecosystem, cause the soil to be poisoned and no longer viable to be cultivated, and livelihood will be destroyed.

[15][16][17] Santos is appreciated with President Rodrigo Duterte's first State of the Nation Address (SONA) as he found out that it is inspiring and encouraging.