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)