Hi Sophie,
1/When we say "he is happy/sad", it means that the person is currently happy/sad. This is the simple present
2/When we say "he was sad/happy", it means that that the person felt happy/sad previously but no longer feels that way. This is the simple past.
3/When we say "he has been happy/sad", it means that the person felt sadness/happiness in the past but we don't know when exactly. It also means that the person doesn't feel sad anymore. This is the present perfect.
The difference is that in the present perfect, the time is not important or known.