Meaning and theme of "On my First Son" by Ben Johnson?

This is a very sad poem, lamenting the loss of a child. Jonson indicates in the title that it is his first son, a "lov'd boy" who was "lent" to him for seven years only, who has died. Jonson bids him farewell, describing him as his "joy" and suggesting, through the semantic field of money and lending, that he has been forced to return his boy as a kind of repayment of a debt -- possibly the debt incurred by his own sin.
However, Jonson goes on to suggest that, perhaps, he should not be sad, but should instead envy the state his boy is now in. He has escaped the "rage" of the world, not least the pain of having to grow old. He commends his child to rest in peace, and hopes that others will think of him as Jonson's best creation, his best "piece of poetry." He finally vows that for the sake of his son, he will try not to become too attached to the things he loves, suggesting that then he will be able to give them away to a better place with God.
The themes of the poem are relatively straightforward. It is a poem about parental love for a child, and especially about the grief that can be incurred when a parent loses a child. However, Jonson's love for his child does not end with the child's death. He still thinks of him, addressing this poem to him and imagining him as a great creation, "poetry," even though he is now lost to the rest of the world.