Hello and thanks for your question.
I appreciate what a difflcult situation you are in but I would strongly advise against keeping your son with you and not returning him to his mother at the end of a contact with you, unless either you and his mother are in agreement, or there is a court order in place.
The only time when you would be justified in keeping your son with you without either agreement or a court order would be in your son was in danger at his mother's home or at immediate risk of serious harm there. In such a case, if you kept your son with you, you would need to make an immediate application to court the next day, and ask the court to confirm that your son could stay with you, at least until the court had had time to look into the situation.
Otherwise, if your son is not returned to her when he should be, your wife could herself make an emergency application to court, and given that from what you say, at least for the last year she has been the children's main carer, the court would immediately order that your son be returned to her. And your case for having your son to live with permanently would be seriously damaged - because the court would judge you not to be a reasonable person.
The legal position regarding children when their parents separate, is that no court order is needed IF THE PARENTS AGREE which parent the children should live with, and how much the other parent should see the children.
If the parents do NOT agree (as in your case), then either parent can apply to the court for a defined contact order and/or a residence order. The wishes and feelings of an 8 year old child will not be the determining factor. The court will make the decision based on which parent the children would be best with, ie considering the abiltity of each parent to look after the children properly, to keep them safe, and to meet all their needs ie emotional, educational and physical. The court will also be concerned to avoid disrupting the children if at all possible eg to avoid the children having to change school. It also rare for a court to order that children should be split up - although it does sometimes happen.
You have done absolutely the right thing in setting up mediation. This is cheaper and more informal than going to court. The family court anyway now requires the parties to have attempted mediation before it will consider an application to court.
However, if mediation does not work, and you do have to apply for a court order, you will need to apply for a residence order on form C100. The court fee is £215. Here's the form:-
I would recommend you get some face-to-face legal advice, even if only for help to complete the court forms, if you decide to go to court without a solicitor. Unfortunately, there is no legal aid any more for this type of court application. Here's where to find a specialist family law solicitor:
I know this wasn't the answer you were hoping to get, but it would be dishonest of me not to set out the legal position as I see it.
I hope this helps and I wish you the best of luck.
Thanks and best wishes...