If you say God had a son , then you imply that God is not infinite or eternal, since the sole purpose or reproduction and having offsring is to ensure survival of your species after you die. So when you say God had a son you are saying God can and will die.

if you say God had a son it implies that he is a being type which uses reproduction in which case you are saying God had a mother and father, So you are saying God was born and not eternal. You also saying that God had a partner, unless you are saying God is asexual. Thus you are saying God is not the almighty and not One, but part of a group of 'almightys' like the avengers.

If you look at humans as an example. When one feels he is powerful and has everything he becomes selfish, withdrawn and looks down as people as though he is better and does not need them. Why then would the Almighty God who does have everything and and does not need us, need to have partners or offspring.

No the sole purpose of Jesus was to be a prophet to portray a message to humans in the form that we can understand. As we would not be able to comprehend or behold God. We can however understand special people who have a strong connection with God who pass the divine knowledge to us. many prophets before him did the same thing hence he would not need 'a son' to do what men before Jesus have done.