The Seventh day Adventists are the ones who really have an issue with the pouring of water or “sprinkling” (which their common misperception is that all Catholics are baptized by sprinkling). What’s at issue is what the root of the word Baptiwo means in Greek. It simply means to wash. Yes, one can wash by immersion, but one can also wash by splashing water over their face, standing under a waterfall or pouring water over their baby in a soothing way.
If you want to do some research, find a good reference library that has a solid collection of Antiquities and Ancient History reference books with pictures. Then find an example of a Greek bath that’s in someone’s home. Now find the scriptures of Paul bapitizing entire households. Did he take them to the river? Undoubtedly not, as Christianity was forbidden in those days and baptisms were held in secret in people’s homes. Did people have rivers in their homes? No. Nor did they have running water. What they did have were shallow pools, roughly 10" deep that were filled occasionally by lugging large amounts of water from a community well and the entire family bathed on that day. In looking at photos of the early baths, it will become obvious that no one was immersed in them…yet they were washed. How would one be washed in so little a depth of water? By pouring it over themselves and cleansing themselves. That is how the custom of pouring water became common place.
Move forward a century or two to the real Roman persecutions. No one dared risk any large celebration of their Christian initiation. These were very solemn events held in very secret locations. Drawing large amounts of water from community wells or from rivers would have been sure to draw undue attention from their persecutors. That’s when small basins of water came into greater acceptance. Some people were not blessed enough to have a sacramental baptism, in the sense that a priest presided. But they were baptized by other members of the Christian community (often deacons) and then the profession of faith would follow months, sometimes years, later in mass gatherings where their Confirmation of their baptismal vows occured. Guess what developed from there.
Throughout the centuries, and particularly in very arid climates, large amounts of clean, safe water were not commonly available to many churches. Yes, the amount of water fluctuated, but the will of the individual did not. Christ did not die so that we could be dunked in a river. He died that we might gain eternal life through our faith in Him and His Father. If our will and intent is truly to become one with Christ, then our baptism is of the Spirit. As noted, your son made that profession of faith at his Confirmation. That was his baptism in the Spirit. If he truly feels he was not baptized, but desires full immersion, then his issue in one of faith in you and your husband, in the Church and in the people who stood as his godparents. If he can see your will and intent that he receive the sacrament of Baptism and understand the duality of water and the Spirit involved in that sacrament and its repeat at the time of his Confirmation, then he’ll understand that he has indeed made the commitment to Christ that we earn through our faith.
Sadly, since the Protestant faiths don’t understand our sacrament of Confirmation, they assume we stop our Christian initiation at Baptism and that it’s the end of our outward profession of faith. It seems your son has likewise overlooked or misunderstood what he did that day and why it’s so profoundly significant. That’s the place for him to start in his own understanding of our faith…his faith…and the faith of the early Christian community.