Research styles of poems. There is no requirement for how a memoriam poem is written, and each type of poetry has its advantages. Whether you choose to write in a fixed form (such as a villanelle) or free verse, the style guides can be altered however you like. While some fixed forms of poetry can help you focus, do not become so fixated on the style that you loose track of your purpose for writing the poem.
Consider your audience. While sharing your memoriam poem is not required, if you are planning to read your work at a service, considering your audience will help you come up with highlights from your son's life that you can note in your poem. You can also reach out to other family members and friends for ideas.
Be realistic in your description of your son and speak from the heart. Elegantmemorials.com notes, it is common for the eulogist to refer to the deceased in a saint-like way. The same can be true of memoriam poems. Elegant Memorials suggests "While we certainly should not dwell solely on the negative aspects of our loved one's life, neither should we avoid making mention of any interesting personality quirks or even significant shortcomings of the recently deceased."
Read drafts of your work aloud. This will help you understand how your poem will sound to readers, and can help you to identify strong points and any areas of awkwardness.