he needs what he needs, and that can only be determined in an interview with the candidate and the priest or whoever he delegates to oversee adult confirmation preparation. Having received 1st communion in second grade does not equate a solid grounding in the faith for an adult, if he received no religious instruction after that time. Classes for adults cover the same topics as those for teens, but geared for their level and learning styles. In this diocese it is a minimum of 2 yrs for teens, and one year for adults, but that is because adults go year-round and teen during the school year only.
On the other hand, a young adult who attended RE all through high school, but did not receive confirmation for whatever reason, may have a good background and only need a review of the doctrine, and actual preparation for the sacrament itself.
But the candidate, not the parent or other relative, has to seek out the priest and inquire, and evidence a desire for the sacrament.
Congratulations for being a good influence on your brother, that is real evangelization.