Its difficult for him to pronounce L because his tongue is short
Because his tongue is short, it’s difficult for him to pronounce L
舌 - tongue - トング
彼の舌 - his tongue
Lの発音 - L pronunciation, to pronounce L
難しいのかもしれません - may be difficult, probably difficult
短いので - because its short
Because his tongue is short, it’s difficult for him to pronounce L