Thanks, Cagey, I think I have got what I wanted to know from your answer.We can't tell much about the boy from that sentence.
"He was a good boy" only tells us that he was good (at least once) in the past.
It doesn't tell us about him. It doesn't answer the other questions you mention: whether he is now dead, or whether he is still alive but isn't a good boy any more, or whether he is still alive and still a good boy. You would have to give that information separately.
The 'was' only puts 'good' in the past. It doesn't apply to the boy.
I hope I understood your question.
Thanks, natkretep, you knew what I meant. thanks again for your answer.For me, it could be the 'goodness' that is in the past or the 'boyhood' or both.
If the good boy grew up to become a good man, you could still say, 'He was a good boy.' Or a good boy who is now a bad (etc) boy; or a good boy who is now a bad (etc) man. Or the boy is now dead.