By literary terms, I take you to mean what are commonly called literary devices, ways to enhance a piece of writing that go beyond bare-bones factual (literal) information. Literary devices make a piece of literature more interesting than simply a dry cataloguing of facts would.
If Jonson were just giving us only the facts, he would tell us he is sad his young son died. He would question why he is so upset when he knows his son is in heaven. He would say his son was a good person. He would determine to be more detached in the future about people he loves so that he wouldn't get so upset again if they die.
That's pretty dull but by using literary devices, Jonson helps us feel his acute, personal grief. For instance, he uses the literary device of apostrophe when he speaks to his dead son. Apostrophe occurs when a poet directly addresses an object or dead or absent person. By opening the poem saying "farewell" to his son, he creates a sense of personal intimacy.
Another literary device Jonson uses is rhyming couplets, such as "boy/joy" and "age/rage." These create a pleasing sense of rhythm and help the poem become easier to remember.
Jonson employs metaphor, a comparison not using the words like or as, when he compares his son to poetry, saying his son was his finest poem or creation. This is a particularly poignant tribute as Jonson was a famous and celebrated poet.