Question

In the trd stanza,the poet says, 'pity but he were a king. ' if a person is a king, why is he to be pitied

Answer 1

It might not be obvious at first, but a king can be and at times should be pitied. A kind bares a huge responsibility on his shoulders, he carries his country, inspires his people. 
Citizens expect so much of their king, and yet the king himself cannot expect much of his people. 
There are rules for the compartment and behaviour of a king. The posture, the language, the manner in which he talks, while the others don't have to carry this burden. 
A king, in my opinion, is anything but free. A king is chained to his throne and is no more free than a man in his dungeons, as all he does is for others and never for oneself.