In most of the cases, it talks about you being either in favor of someone or against. There's no any middle-way i.e. third choice.
"Your either for or against him"
Isn't it should be "You are either in favor or against him. Why does it use your? Why omit in favor as well? How can you be for someone in this context? A girl telling I'm for you may express her love but it does not necessarily mean in favor (opposite to against) I guess.
We generally paraphrase either...or this way
You go to the railway station OR
You go to the airport
= You either go to the railway station or airport.
So, in above case...
Your(?) for him OR
Your(?) against him
Your either for or against him
After Helix's comment
Even if I consider that as a typo, what about the second question?
Does You are for him mean You are in favor of him?