Since your family keeps contacting them, they are probably afraid that you will ask for the dog back. I'm sorry you had to lose your dog, but he is THEIR dog now. Since they kept in touch for as long as they did, they probably are a very nice family for the dog. Only considerate folks would do that.
But now, if you are trying to contact them "in every sort of way", that could legally be classified as harassment. Quit pestering them.
EDIT:
"I just miss him a lot (for everyone who said "he is their dog now do not interfere") and was wondering if I could visit him."
Benjamin, this is precisely what the new owners are afraid of. If you visit him, they're afraid he will want to go home with you, since he's known and loved you for so much longer then he has known and loved them. Most important, if you visit, it will confuse the dog. He won't know *who* his master is--you or them? It will be a setback in his adjustment to his new home. You don't want to stress him out any more than he already has been, do you? And it's not as if it would help *you*. You'd just feel awful leaving him again, and he would know that and feel awful too.
Sometimes love involves self-sacrifice, and this is one of those times. I know it's tough. But for your dog's sake, let go and allow him to settle in nicely with his new family.