The correct word to use in this sentence is
pertinaciously.
“My brother clung PERTINACIOUSLY to his belief
that Alfre Hitchwingspan was the greatest film director of all time. “
A person who is pertinacious is someone who holds
tenaciously to his own beliefs or opinions. When a person has this type of
attitude he/she is usually perceived as being stubborn or obstinate. Being pertinacious
could also mean that someone is extremely persistent and that no amount of
recourse could change his beliefs.
