The thing to remember may1day is that the court will always take the interest of the child as paramount which you as a grandparent would want. There is nothing to stop you applying to the family court for leave to make an application for a contact order you would probably need to show the court that it would be in the child’s interest for you to retain contact. If it is granted any problems with exercising your contact rights report to the court it may in the fullness of time result in a penal notice being attached to the contact order.
Your son as one of the parents does not need to apply to the court for leave to make an application for a contact order the application is his by right. I am sure he knows it will be important to always be reasonable in his dealings with the other side English law likes reasonableness. It need not cost you or your son a fortune.