Two words: Sandy Hook.
It's natural to want to protect our wonderful children even if they do something that's wrong or against the rules. But every child has a right to feel safe.
The issue isn't the fact that he made something out of paper. This issue is, it was a gun. If paper guns are allow, what about plastic water pistols? What about plastic guns that look real, the kid that get kids shot by police? What about Grandpa's WWII "souvenir" relic? Dad's loaded handgun because "the safety is on." Where does it end? You are seeing this only from your child's perspective, and not from the perspective of other children who might see some form of a gun and be scared. Or a real gun and be so desensitized that they don't alert someone or stay away from it. You really want children to recoil at the sight of any gun and let an adult know.
So this is a teaching opportunity for you, and of him. First off, "ignorance of the law is no excuse" and this will encourage both of you to learn the rules of any situation rather than assume that anything is okay. How many adults use that argument to say, for example, that they didn't know the speed limit and didn't see a sign? So an 8 year old learning that there are rules is okay, and admirable. If there are on line policies or a handbook and parents didn't take time to read it, that's not the school's fault.
Secondly, you can sympathize with your son's disappointment and still understand the other perspective. I suggest you encourage your child to not just see things from his own point of view. He's only seeing that he was "booed and hissed" (which may or may not be an accurate assessment) and not that other kids knew it was unacceptable behavior. Perhaps the other kids, if they did boo, were dealt with separately. You really don't know. You see only that he might be bullied, and not that his behavior may have been considered bullying or threatening to others. You can't say that one is unreasonable and the other is not. And kids do stuff when there's a sub - they just do. They play pranks, they boo, they even make toy guns out of paper. There's a whole dynamic to it, and if you understood 8 year olds and classroom dynamics, you'd be able to see it.
Third, you have to get past the idea that your son will be damaged for life for this. Of course it's not going on his transcript! Of course colleges don't give a damn about a kid being sent to the principal's office. And the school has no plans to torture your child - they are not going to deprive him of food and water, for heaven's sake. He'll be able to get up and stretch, go to the bathroom, and so on. Where did you get that idea?? Are you passing your anxiety on to your child?? You really have to model calmer behavior for him - he's going to pick up on your anger. I'm sure he will sit at a desk or table with a folder of his regular work from his class, and he will just miss out on the socializing in class, lunch and recess. He's not going to be blackballed forever. In fact, half the kids will think he was out sick or dismissed to go to the doctor. If he is bullied, that behavior can be dealt with
The fact is, your son will learn from this, he will (with your help) understand that every child has a right to feel safe, and that guns in one form can lead to guns in another form. He also needs to understand that rules are rules, he's responsible for learning and following them, and that he can question a regulation by asking about it or writing letters to the principal saying he disagrees, but he cannot just defy them.
Please try to see the learning potential here, that the rules are there to protect your son rather than to make his life miserable. He will, I hope, understand things and never do this again, in which case the one day in the office will have totally served its purpose.
Take a breath, settle down, and tell your son he has to take his medicine. While he's sitting there, he can take a break and think about why guns are scary to others and why they should be scary to him too.