After carefully reflecting on this thread I came to the following conclusions;
What you and your husband have in common;
-Belief in a merciful God
-The desire to raise your child with virtues, kindness, patience, caring etc.
You could easily agree to raise your child on the common points.
What you disagree on;
You believe in a Zoroastrian God concept, your husband believes in a trinitarian God concept.
You could tell your son that you have the Zoroastrian understanding of God and his Father has the Christian concept.
This way he will already understand that there is more than one way to think about God. He will almost inevitably seek to understand the two when he is ready, but on the other hand if he never feels ready then I guess that would be his choice.
In this way I think your difficulties will be solved, because you have enough in common to raise him on, and as far as the differences go the ball will be in your son's court.
If your son asks you to explain the Zoroastrian concept (he almost certainly would ask either you or someone else at some stage), then you could explain it to him since it would be his choice to raise the question.
Alternatively if you work up the courage to discuss it, your husband
might pleasantly surprise you by agreeing that he learn both concepts right from the start, I guess if you don't ask you can't win