He asks: “Was it prior life trauma that caused the vulnerability in my son to cause the stuttering he now has? Or was it a mercury toxicity early on in his life?”

There is almost always a karmic component to difficulties within the body, whether physical or emotional. In this case, there is a combination of karmic vulnerability and corruption by spirit meddlers prone to produce stuttering in their host. And that became a kind of competition between two competing spirit meddlers to see who could win the race to take over control. That simply adds intensity to the formation of the problem and its persistence because there will be a conscious teaching and reinforcement for the body, perceiving that the individual themselves wants their speech to be delivered in a staggered fashion and will work to see that happens. So this legacy needs to be undone now, in order to restore smooth functioning for him. This will take time and effort, given the long-term experiencing causing it to be quite grooved in.