I don’t think anyone will care if it was an official visit or not. He can mention that he visited with his brother somewhere in the application or in an email to admissions.
FWIW my D applied to the college my S attended. She did not do an official visit since we were on campus many times and she got more information from her brother than she would get on any tour. In her “why X college” essay she mentioned the situation and it didn’t seem to be an issue (she got in but attended elsewhere). So official visits are not always needed, especially when there are other significant displays of interest – and your S sounds like he is doing everything right.