The sky is falling beneath his feet.
In question: The foundation on which he stands is shaky. He begins to lose stability.
(Modern Hebrew)
Translation
Expression (Hebrew)
הַקַּרְקַע נִשְׁמֶטֶת מִתַּחַת לְרַגְלָיו
Definition (Hebrew)
בְּהַשְׁאָלָה: הַבָּסִיס עָלָיו הוּא עוֹמֵד מִתְעַרְעֵר. הוּא מַתְחִיל לְאַבֵּד יַצִּיבוּת.
(הָעִבְרִית הַחֲדָשָׁה)