I was born and raised in St. Louis, Mo., and except for a brief time in Biloxi, lived there until 1993, when we moved down to Ft. Myers, Fl. Nothing will drag me back. Yes, it is hot in the summer, but it wasn't Floridians who complained about the 100+ temps this year. Or a lot of years. People who live down here year around know how to deal with the heat.
As far as season changing, and missing those "glorious colors", something is always in bloom, and we have reds, golds, purples, oranges, greens, pinks, oranges....all kinds of glorious colors blooming. We get cooler weather, which is one reason they sell heaters in the stores here, lol. We do get hurricanes, tho we haven't had any major storms lately. But look at what the rest of this country has had this year.
Maybe the thing for your SIL to do is rent for a couple of months before making the move with the entire family. He might be more than pleasantly surprised.