ציטירנדיקער פּירוש אויף קהלת 2:18

Under the sun. תַּחַת [meaning] in exchange for the Torah.²See above 2:18.

Finally, Divrei HaYamim II 11 describes Rechav'am's successful efforts to build and fortify the cities of Yehudah. He even manages to oversee two Shevatim defecting from the northern secession and rejoining his kingdom, Binyamin and Levi. All in all, it seems that Rechav'am is not at all a fool. Indeed, it would be utterly shocking to find the son of Shlomo Hamelech to be a complete fool. Kohelet 2:18-19, which speaks of the author’s son as a fool, speaks hypothetically and does not describe reality, as noted by Da’at Mikra ad. loc.