Yes, Freya and Maddyone, you are quite right. Legally it is abduction. However, unless there is a situation whereby the child is not going to be returned or the child is in danger, I suspect that the police would not do much until after the return date.
I suspect that making a big thing of this on its own, without starting the route of court defined access, will set up an even bigger risk. If the ex won't go to mediation, then the courts will see that there is a problem and will be more than happy to help come to a suitable arrangement. My son had to do this with his ex partner. If your son is on the certificate, he has parental rights. Today the courts don't talk in terms of custody, they talk about the resident parent and without a court order, if your son decided to keep his son after an access visit and not return him, whilst an ill-advised thing to do, the ex wife would have to go to court to get the child back. It just isn't as clear cut as it used to be but as a child who was abducted by her father for over a year, let me tell you the repercussions from that action is horrendous and effects inter family relationships for a lifetime.
Your son's soliticitor is quite right, legal action won't necessarily make his ex change. There is always illness which can be used to halt a child's visit and a host of other excuses which would mean a load of court appearances with everyone getting more bitter as time goes by. Far better to try to resolve this with as little acrimony as possible taking the view that it is the child that matters most into account. Sometimes it just takes a father to ask how he can help make things easier for the wife in arranging access rather than just demanding their "rights".