What do you tell your child when the other parent shows no interest in him? The answer depends on the age of the child and whether the child was inquiring. If the child is young and wants to know "Why?" look for the least painful truth.
Here are some reasons that fathers (and occasionally mothers) don't make contact with their children. The parent may simply be selfish and irresponsible. In that case, a parent could say, "You are the best child a parent could ask for. It's not your fault! Daddy (or Mommy) isn't thinking too clearly right now. He's lost his way for a while, but later, when he finds it again, he'll want to see you more."
In some cases, parents are mentally ill. They may have broken under the pressure of the divorce. If that's the case, say, "Mommy (or Daddy) is sick in a special way that makes her unable to think right. Let's hope she gets better soon. When she does, she'll want to see you more!"
If a child didn't ask, but I sensed that a parent's absence was troubling him, I would ask how he's feeling about it. If he says, "I feel bad," I would say, "That's the way I feel too. I'm sorry you're hurting." Then I would hug him. After allowing him to say all he wants, I might share some of the previous statements. Then I ask him why he thinks the other parent isn't coming around more.
It is tricky, but you still want to avoid criticizing the other parent while still conveying that the child is lovable, and that the problem is not his fault. You will need to sharpen your diplomatic skills!
For an older kid, ask him about his feelings and then simply listen. Give him permission to feel, then suggest he write his feelings in a letter and send them to the neglectful parent. It may solicit a positive response. Or, sad as it is, you may verify that there isn't much feeling there for the child.
If there is little affection evident, then you'll have to deal with the consequential pain. Most of us spend a lifetime trying to verify, one way or the other, that our parents love us. You may have to say things like "Your father just doesn't have it in him. It's not you — it's everybody. He just doesn't know how to love."
Stay away from ugly language. But if the child uses it (except for profanity), permit it for a while. Then encourage him to forgive the neglectful parent. Just don't force the forgiveness issue on him immediately. He will have to get over his anger first, and that may take a while, as you well know. Be patient.