I have joint custody w/my ex husband but he has primary physical custody of my son. He may not be the father but he doesn't know that. My son is 10yrs. Now and I want him to come back home but I know there is going to be a custody fight should I bring this up?

Your child is going to be torn... he grew up knowing that your ex is his dad. Your ex might ask for a paternity test. What do you think is the best for your child? Your ex's care or in your care? Your ex may give a fight if he thinks that the child is better off with him, he might petition for adoption. Also depends on the law in your state or province; most government will focus on your child. Whatever the outcome is your child will be affected.

If the child was born during the marriage then it doesn't matter who's DNA is mixed into the child. The legal father of the child is the husband of the relationship. Threatening to destroy that will only bring you trouble and extreme costs.