His boss, Michael, or his boss Michael--which is correct?

When referring to a title followed by the individual who holds that title, should I use a comma? For example, should I say:

At the end of a sentence "his boss, Michael." or "his boss Michael.

At the start of a sentence "The writer, Michael," or "The writer Michael"

Titles like this are appositives: a construction where one noun phrase identifies an adjacent. Your question on comma use is almost a duplicate of this one, but a little broader, so I'll try to answer it in full. I'll quote the same style guideline as the top answer there:

Commas are used to set off an appositive when the appositive can only refer to a specific item. (The Chicago Manual of Style, 5:21,123; 6:22-24). Examples:

Mary's son, Jesus, is thought by many to be the Messiah. [Mary had only one son]

Dostoevsky's book Crime and Punishment is the great books of all time. [Dostoevsky wrote more than one book]

Your sentences fall into the same two categories: presumably, Michael has only one boss, and there are multiple writers. Thus we would write

His boss, Michael, ...

The writer Michael...