You are right that the sentence was not correct. The speaker was trying to use subjunctive mood - an excellent idea - but didn't know how to do it right. This is what he was trying to say: "I'd prefer that he spend more time at school rather than be on his phone"

Below, ‘that’ and the second ‘time’ are optional. I’d prefer (that) he spend more time at school than on his phone. (More than 50%) I’d prefer (that) he spend more time at school and less (time) on his phone. (Increase his time time at school, decrease time on the phone) I’d prefer him spending more time at school and less (time) on his phone. (Expressing a desire for an increase without using the subjunctive)

A point the other answers have not made : you don't need 'rather', 'being' or 'be' before or after 'than' in 'rather than' or 'than be' or 'than being' ; in all cases you can simply say '. . he spend /to spend /he spent more time at school than on his phone'.

Hi Arti. To me it sounds like the speaker may have mixed two grammatical structures. ⭐️⭐️would rather + subject + past tense ⭐️⭐️ 📌 This means we would prefer somebody to do something. 📚Example: I'd rather he spent more time at school than being on his phone ⭐️⭐️would prefer + object pronoun + to-infinitive ⭐️⭐️ 📚Example (your sentence): I'd prefer him to spend more time at school rather than being on his phone. ⭐️⭐️prefer + that + subject + bare infinitive ⭐️⭐️ 📌 This is the subjunctive mood which is very old-fashioned and formal. 📌Many languages (like your mother tongue: Il congiuntivo) use the subjunctive to talk about things which are possible, desirable or imaginary. 📚Example: I'd prefer that he spend more time at school rather than be on his phone. ❗️This outcome is desirable NOTE: The subjunctive in modern English is very rare!

Hi Arti! You are right , the correct sentence is " I would prefer him to spend more time at school rather than being on his phone."