We go every year in mid-to late August. It is HOT, but that just means we take it easy and spend a lot of time at the pool in the afternoon. I find the crowds thin out a little towards the end of the month as many schools are starting earlier and earlier each year. In fact we are going this year and staying at the BCV. We stayed at the BC on our honeymoon six years ago and found that we loved being so close to EPOCT and the Boardwalk. We'd stroll around the World Showcase in the evening and have drinks in a few of the countries and then go over to the boardwalk and go either to the ESPN club or Jellyrolls.
I guess it really depends on what your brother would like to do. If he's into the park thing than he'll enjoy the BCV or BWV since they're so close to Epcot and MGM as well as offering adult fun at the boardwalk. But, if he wouldn't enjoy the parks and would enjoy a beach vacation more, than I would book Vero. Although, I can't imagine baking in the sun on the beach on a hot August day.