He has a bug in his head
(slang) figuratively: he has started to engage in a new or unusual field in a stubborn, obsessive way.
(Modern Hebrew)
Translation
Expression (Hebrew)
יֵשׁ לוֹ ג'וּק בָּרֹאשׁ
Definition (Hebrew)
(סְלָאנֶג) בְּהַשְׁאָלָה: הֵחֵל לַעֲסֹוק בִּתְחוּם חָדָשׁ אוֹ מוּזָר בְּצוּרָה עַקְשָׁנִית.
(הָעִבְרִית הַחֲדָשָׁה)