To be honest he has every right to make jokes like that even if they aren't in good taste. Paranoia is going too far, especially when it's dealing with kids. Those concerned about school shootings need to work on modifying the environment that causes them rather than going after any kind of violent behavior or expression.
That said, situations like this tend to be somewhat loaded because we do not know the entire story. For the most part law enforcement cannot reveal what information they have or uncovered to the media. Active investigations are usually kept under tight wraps, and the facts they have tend to only be revealed during trial. This does not however apply to the other side of most criminal cases, meaning that outraged mothers and such who might have no idea what the police have actually got and go QQ to the media and rally tons of support and put pressure on the process when it might be entirely misplaced.
I say this because right now all we know is that the police didn't immediately search his living quarters. We do not know what he said to the police during questioning. We do not know what they might have found in remotely accessed E-mail (which might not even require his computer) or what was said on Facebook. We do not know if the police found and searched storage areas he might have had outside of his house.
To put things into hypothetical context if the kid was found to be planning a massacre with the info on the internet somewhere, and the police say raided a collection of guns and pipe bombs the kid was storing in a locker at a bus station, bowling alley, or whatever else, you might not ever hear about that since the police aren't generally going to tell the media unless the media finds out on it's own.
We do not know what the police have here, and to be honest if the prosecution is pushing the case and 8 years is the deal they are willing to offer on a plea, I think they have more than we're hearing. The authorities aren't going to push a case over a stupid internet joke that will cost them tons of money and then get thrown out by a Jury of his peers most of who would probably go "WTF, right to free speech and all that, why are we wasting time on something so stupid?".
The kid getting beat up in prison tends to make me think that there was something bad going on here too. Inmates tend to have their ways of finding out why people are in prison, and what they are accused of, no matter how secret they might try and keep it. Inmates also don't generally like people who do things to kids. It makes me think that he was probably planning on doing something for real, and that there is enough tangible evidence where the prosecution is confidant enough to bring the case before a jury.
That's my thoughts at any rate. I won't be signing this guy's petition. As far as I'm concerned he'll have his day in court, and if it's reported on, I'll give my opinion based on what the prosecution reveals. If this is about nothing more than stupid internet comments the kid should not only go free, but launch a huge law suit. Heck, by that logic they should arrest me for talking about mass murder due to some of my angry political rants. If they have some serious stuff on him though... well, I probably won't be very sympathetic, and just looking at this it seems to me that there is a lot going on here that we aren't seeing.