Your child's feeling he cannot live without a relationship is his perception of how he would be without the relationship. That is just how it is from his point of view. When you see this sign in your child, he perceives the relationship he lost was important, thinks it is gone, and believes he cannot live without it. It does not matter how others see or think about him and the relationship. His perception is his reality and the basis for his actions.
Start with his reality, with his perceptions. It is easier to unilaterally decide what he thinks and feels are not valid, the relationship was not actually so important, or he has not really lost the relationship. You are tempted to say to him, "Things will work out." Your point is his perception is wrong.
Your helping him starts with adopting his perspective, his perception. Why does he think the relationship was so important? What makes him think he lost the relationship; and most importantly, why does he think he cannot live without it?
Your child is best served if you start the conversation like this. "I want to understand. Please help me understand. What about the relationship was so important for you? What have you lost that is so very important to you? Will you talk to me about what you're thinking and feeling?"
Your child's grief, anger, fear, and emptiness are real and painful. His loss is real; and living past the pain feels impossible to him. To help, share his grief, his strong feelings, and his pain. It is as if you take part of it into yourself. Your child cannot handle it by himself; but together, you can.
Here is how to tell if you are helping. Can you feel his loss, his emptiness, his grief, and his pain? Is it a little as if the feelings were yours? If so, you have achieved empathy. That is the level at which real help and healing for your child can begin.
Refrain from telling him how he should think or feel, and more importantly, from saying his feelings and how he thinks about what happened are wrong. Listen and feel until empathy comes for you. When it does, you can then honestly say, "I'm afraid for you. I'm afraid for me. Maybe I don't totally understand; but I feel awful and hurt as if it happened to me. I want to be close to you and help you get through this. Can I share your grief with you and struggle through it with you?" Holding or touching your child may make him and you feel better; but holding him emotionally is the key to really helping him.