Sorry, I need more information like what subject the boy studied and how he studied to answer your question. Do you mean the following?
- هل درس الولد بجد الإنجليزية؟ (Did the boy study English seriously?)
- هل درس الولد بجد الرياضيات؟ (Did the boy study math seriously?)
- هل درس الولد بجد التاريخ؟ (Did the boy study history seriously?)
- هل درس الولد بجد العلوم؟ (Did the boy study science seriously?)
If the boy studied seriously, you can say "نعم، درس بجد" (Yes, he studied seriously.)
If the boy didn't study seriously, you can say "لا، لم يدرس بجد" (No, he didn't study seriously.)