you are siding with him.
you are siding with him.

is this sentence written correctly? i heard it in an american drama.

or you are taking his side.

Question

you are siding with him.  
you are siding with him.

is this sentence written correctly? i heard it in an american drama.

or you are taking his side.

Answer

Yes, "you are siding with him" is correct. The verb "to side (with)" means to be on the same side or take the same side as soeone else.