Legal Question in Family Law in California

If you have no existing court orders, you don't need any written permission from court or mother. Your arrangement can be totally informal. It is, however, always wise to put any agreements regarding custody and visitation plans in writing in some form, even if it is just a confirming email. The only point at which you have to have a formal written document is if the plan differs from an existing order, or to leave the country.

You don't need to petition the court for visitation before taking your son home with you, but you will want everything in writing. You may eventually wish to open a court case establishing formal orders granting you visitation and custody rights. This prevents any control the mother might have over you in regards to denying you visitation, putting requirements on visitation, etc.