Hi,
1. Is he studying right now? OK. - Yes, he is.
2. Does he study math now? OK. Yes, he does.
3. Has he been studying today? OK. - Yes, he has.
4. Had he studied yesterday? OK. -Yes, he had. However, you'd much more commonly hear 'Did he study yesterday?'
5. Could he study at this moment? OK- Yes, he could. However, you'd much more commonly hear 'Can he study right now.
Best wishes, Clive