On May 28 in Boston, Thompson will learn whether he has won the National Cartoonists Society’s Reuben Award for cartoonist of the year. It is his second straight nomination for a strip that was syndicated almost four years ago. “Cul de Sac” is carried by nearly 150 newspapers, including The Washington Post, where it began. It has spawned four books, a handful of animated shorts — and legions of fans.