Er ist mit seinem Vater

What makes this dative and not accusative. I would have used seinen but apparently that is incorrect, any help?

9/4/2012, 8:25:21 PM


"mit" requires the dative

9/4/2012, 8:49:39 PM

with 'mit' we use the dative ^^

9/5/2012, 2:45:03 PM
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