Translation:I am going to go to the theater with him tomorrow.
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.