Just because his parents went to a Mormon church while they lived in Las Vegas, doesn’t mean they converted to Mormonism. I am Catholic and If I go to a Lutheran church, that does not make me a Lutheran. And since Rubio is Catholic he most likely attend Mass in a Catholic Church in Miami
“Rubio’s parents baptized him Catholic and he is now a practicing Catholic, but when he was 8, his family moved from South Florida to Las Vegas, where his mother attributed the wholesomeness of the neighborhood to the influence of the Mormon church. Young Rubio was baptized again, this time in the Church of Jesus Christ of Latter-day Saints. He spent three years as a Mormon, upheld its teachings more enthusiastically than his parents, and chided his father for working as a bartender, a no-no for Mormons who abstain from alcohol.”
http://www.religionnews.com/2015/04/13/five-faith-facts-marco-rubio-catholic-always-catholic/
“Rubio and his wife Jeanette often visit Miami’s Christ Fellowship, a Southern Baptist congregation the couple appreciates for its strong preaching and children’s programs. Rubio has donated at least $50,000 to the church, which he attended almost exclusively from 2000 to 2004. But he now finds his religious home in Catholic churches in Washington, D.C., and Florida. In his memoir, Rubio writes that he will go with his family to Christ Fellowship on Saturday nights, and Mass on Sundays at St. Louis Catholic Church. His children have received first Holy Communion.”
http://www.religionnews.com/2015/04/13/five-faith-facts-marco-rubio-catholic-always-catholic/
Wouldn’t allowing your son to be a baptized Mormon indicate thet they at least believed in Mormon dogma? I can assure you that there are many RC churches in Vegas and have been for decades. Of course the RC church would have first claim on his soul. He however, as noted, was a true Mormon believer at the age of 8.