Irrelevant.
It isn’t the guilt that hurts the worst.
It is the longing for your child.
You don’t “let go” of a longing for lost family or loved ones. Forgiveness isn’t the biggest problem.
You better read Philippians 3:7-10 again. It is ALL about suffering loss, going through a grieving process to closure, acceptance, and letting go, and at the end finding Christ has filled that area of loss, leaving you better off than you were before your loss.
Even medically it has been shown you'll live longer and healthier if you go through the grieving process to closure which oftentimes DOES contain giving and/ or receiving forgiveness. The temporary pain of processing though grief to closure is not as bad as the permanent pain and suffering of holding on to regrets and the bitterness of loss.
Actually, forgiveness may be what he needs the most.
Forgiving HIMSELF.