If you are answering a question, the answer would be: 'It's him.'
'which man is your neighbour?' 'It's him.' (Pointing).
Sometimes someone might exclaim; 'It's him!,' when they see a person. There is an implied question, 'who is that?'
'It;s he....' is always the start of a statement: 'It's he who will pay for the tickets.'
He is the subject, 'him' is the object.