How to get Full custody of my son ?

The only way is going through court proceedings and proving that he is unfit as a person to look after your son. If that does not work you have no more options. I do not know your location but this will probably apply to the USA and definately the UK... but by law you need to give custody to him for at least 2 days a week or you can face criminal proceedings although a criminal charge is unlikely and is usually sorted in a family court. The only way to get custody is to proove he is unfit as a parent.