Here is my take.
This is Hebrew parallelism. The "dream" and "word" are both messages from God to the prophet that are intended to be told to others.
Both the dream and the word are "in" the prophet. The dream and word are not "with" the prophet.
The προς αυτόν is the accusative object. I would take that to be the object of "tell", (to him), the neighbor of the previous verse.
who devise that men may forget my law by their dreams, which they have told every one to his neighbour ...
Let the prophet
in whom there is the dream
tell his dream [to his neighbor, from the
in whom my word is
to him [ie neighbor] tell
my word in truth.
If you say the word is "with" the prophet, there is no parallel to John 1:1. There the word is προς God, not “with” a prophet of God.
Oh my ! αυτόν as in πρὸς αὐτόν is the ὁ προφήτης.... The verse is saying
".... and the prophet that hath my Word with him (meaning the prophet),.." In other words, the prophet who is in custody of God's Word.