My experience with RCIA is that converts, primarily those who are not entering due to marriage, do not join to become protestants.
They join because they have an impression of the "Ideal Church".
A lot of protestants going through the program are disillusioned after a few weeks but continue to march through the process and listen to baloney spewled by the RCIA director and team members. More conservative sponsors sit back tight lipped knowing that they will get reamed if they stand up for the truth.
Heck, in the last NO parish I was at I was made to sigh a contract agreeing that I was not allowed to "teach". Here I am a person who had sponsored others and have about 5 years experience with team on RCIA and been a Catholic, baptized as an infant, for nearly 5 decades and I am not allowed to "teach" the person I am sponsoring.
Tell your brother to parish shop and interview the RCIA director and get a finger on the way the priest thinks.
Make sure he is not getting a watered down version of Protestantism or a shake and bake RCIA program.