Yarnū is an imperfect verb from r-n-w (ر ن و), “to gaze with desire or attention.” Al-ḥalīm is the nominative subject, while the fronted phrase ilā mithlihā foregrounds the object of his attention.
Ṣabābatan is accusative and expresses longing or emotional attraction. It turns the action from mere looking into a response charged with feeling. Idhā mā frames the time or circumstance in which the attraction becomes noticeable.
The order is focus of attention → observer → inward feeling. Grammatically and rhetorically, the portrait shifts from visible features toward their impact upon a character presumed to possess self-command.