He's concerned about secular influence and wishes to attend a yeshiva abroad.

That's not a reason to leave Israel; there are many good Yeshivos in Israel that are internally religious and maintain strict dress codes. This helps guard against the influence of the Yetzer Hara. While in the beginning, the Yetzer Hara convinces you with smooth talk, the end is bitter https://www.torahrecordings.com/rebbe/igroskodesh/011/008/3488