The first thing is to reestablish your son's contact with his son, as per the court order. So I would suggest that your son either himself or his solicitor writes a very polite letter to the mother's parents with a copy of the court order, pointing out that the court has said that he can have fortnightly contact with his son, and when can that restart.
If the mother's parents still refuse to allow him any contact with his son, then he can apply to court for the contact order to be enforced.
With regard to having his son come to live him, your son needs to apply for a residence order. If granted, teh resodence roder will state that your grabndson (named) is to live with his father (named) . It isn't automaric that court will agree to grant a residence order in favour of your son, because the court will look at what is best for the child, rather than assume that he will be best off with his father.
So your son will need to show that he has suitable accommodation available, that he is able to look after his son well, especialy given his special needs, and that his son would be happy with him. If the mother's parents challenge this, or were named as the child's guardians in her will, the court will have to decide whether your grandson would be better off with his father, or better of with the mother's parents.. It will certainly go against the mother's parents if they continue to refuse to allow your son any contact, as the family court is keen that children should have contact with a parent that they don't live with - unless there is very good reason why not.
Unfortunately there is no longer any legal aid for this type of application to court, but even so, I owudl stringly advise your son to get soem face-to-face legal advice from a solicitor, even if just to fill in the court forms, if he decides to go to court alone.
The court fee is £215 to apply for a residence order, and £215 to apply to enforce a contact order.
Your son will need form C100 for the residence order application, here:-
and form C79 to enforce the contact order, here:-
Here's where to find a specialist family law solicitor for some face-to-face legal advice & help with the forms at least:-
The fmaily court does now require the parties to have attempted mediation, before it will consider an application to court, so even if your son knows in advance that the mother's parents won't go, he needs to be able to show the court that he has tried mediation. Mediattion is a round-the-table discussion with a trained and neutral mediator who tries to help the parties to reach a fair and workable compromise.
Here's where to find a family law mediation service near to your son:
I hope this helps and I wish you the best of luck.
Thanks and best wishes...