"As someone with Schizoid Personality diagnosis, and as an avid tabletop roleplayer, I do have a fair bit of experience with the exploration of alternate realities both as a method of personal escapism and as creative entertainment. As such, I wanted to offer a few observations which may or may not help.
Without knowing more about this other reality he's dealing with it's difficult to understand where his inspirations might be coming from, so I'll try to keep the advice general.
First of all, in his mind he's a combatant, trying to provide a refugee service. So, what he needs are allies, not opposition. If you accept his reality as true, then you can become his ally in this situation.
From that position of influence, you might be able to channel his efforts into something more constructive. For example, ask if there are other ways in which he can help these people without the weapons. They just need to escape the war in their reality right? So it's actually more of a refugee situation than a military one.
What I'm suggesting is that you engage with this reality, as a way to steer things into a productive and less aggressive outcome.
As for the issue of weapons, you could try to point out the technological issues involved which might explain why these weapons are failing to function even once printed. This can work into highlighting how the two worlds differ, and as such he might have to find alternative ways to help.
By the sounds of it, he doesn't actually want to bring the war here, he wants people to escape the war there, so maybe there are ways he can help hide the people instead of propagating the violence.
The thing about altered perceptions of reality is that they are not always bad, and in fact they are not always entirely wrong as they can be a way to reinterpret things within our own reality.
To help someone with reality perception differences, you have to really understand things from their point of view before you can offer helpful guidance. In the case of your son, he's trying to help people, and that's a noble task.
Even if you cannot perceive of these people, he can, and it's very distressing for him to sit idle while others suffer. Sounds like he's a good person who just needs to find a way to help which doesn't conflict too badly with our perception of reality.
As for the pursuit of gaming, such as tabletop gaming, I'm not comfortable suggesting that. Roleplaying has attracted terrible negative press in the past when people with reality perception issues adopted the fantasy world as reality.
As he is already struggling with that issue, I'm not sure that delving into more realities will have a positive outcome. However, one thing Roleplaying does teach is the compartmentalisation of differing realities, so perhaps raise the subject with his medical professional and see what they say about it."
