What is the person's name? Jos?
And I think you mean apostrophe, right? Not comma?
If I wanted to say--This is Jos's backpack. Would the comma go between the s's as I have included, or would there just be a comma after the s?
2 answers
It's an apostrophe, not a comma.
If the boy's name is Jos -- then it would be Jos's backpack.
If the name is Joss, then you'd write it this way: Joss' backpack.
If the boy's name is Jos -- then it would be Jos's backpack.
If the name is Joss, then you'd write it this way: Joss' backpack.