If a parent won’t let a child of record be baptized, how do we teach him about the Holy Ghost?
If you are the parent of the child, you teach him about the Holy Ghost the same way you teach him about baptism or priesthood or the Atonement: through Family Home Evenings, daily devotionals, impromptu testimonies, and practicing gospel living. The child is still subject to the light of Christ, and the primary difference (in my understanding) between a person who has a godly walk and that same person with the Gift of the Holy Ghost is that the confirmed person is a member of Christ’s church and can be sanctified. The fruits of the spirit are found in discipleship and will be available to him, although the gifts of the Spirit will be limited (as they were with Cornelius).
The greater concern is why the parent won’t consent to the baptism. If the parent is a member, he or she is under obligation to teach the child baptism at the age of accountability or the child’s sins are upon the parents. If the the parent is not a member, I would hope there’s a pre-18 age set where the boy can join in full fellowship with the faith of his choice. – Why not 8? at that point boys across the US are trusted to make and keep oaths in cub scouts.