Welcome, Beautiful Ladie,
I'm sorry, but I have to disagree with your brother's doctor. I've had an ileostomy for 5 years and go swimming all the time in the ocean, public pool, and at waterparks, and have never had my flange fall off or been sick with an infection.
Can your brother take a bath without the flange falling off? If not, you should shop around for a better product. The flange I use is the Coloplast Assura extended wear #2833 with cloth covered pouch #13986. It has never leaked or fallen off in 5 years, and I only have to change it once a week.