Well if no-one can contact him and his phone is 'switched off' then perhaps something has happened to him. In all likely-hood nothing has, and you have every right to be upset for your son, and there probably isn't a very good reason, but you don't know that yet. He lives at home with his parents, yet they haven't seen him all week and his phone is not working...
Wait till you calm down and then write a letter to your ex saying how very disappointed his son was that he wasn't there as had be arranged. Ask him to let you know as early as he can in advance in future if he has to cancel or change arrangements.
Remind him of his obligations and duties to your son, and how it is emotionally harmful for you son to be put through this kind of disappointment and rejection.
Perhaps if would be worth speaking to your doctor or son's school to see if they have a councellor or someone he can speak to in confidence about his feelings on all this. It might help him deal with his feelings to be able to express himself rather than keeping it bottled up.
Keep up the relationship with his grandparents though, that's invaluable and will make him feel part of a wider family. Perhaps you could arrange with them to visit just them (or them come to you), or perhaps go on an outing together (bowling, park, zoo) so your son can enjoy seeing them without being disappointed that his dad isn't there. Get your son used to have a relationship with his grandparents separately from his dad.
Keep strong for your son
best of luck
Gilly