Translation:I am going to go to the theater with him tomorrow.
I see no difference between 'I am going to go ... tomorrow' and 'I am going ... tomorrow', other than the second one is a more closer translation than the first, and isn't accepted
(presuming you meant 'to the theater')... this sounds awkward in English. You are expressing an intention in the future, so the usual construction is to use the present continuous tense of the verb. The present simple tense is used for habitual actions.
As a native English speaker, this is completely wrong. It is perfectly natural to use the simple present tense to refer to a future event, as this Wikipedia page indicates: https://en.wikipedia.org/wiki/Simple_present It lists examples such as:
We leave for Berlin tomorrow at 1 pm.
Our holiday starts on the 20th May.
None of these would sound awkward in English at all.