Is it normal to worry about a friend this much?

Yes this is perfectly normal, it's somebody you care about and because of the distance you can never be 100% sure if they're secure or safe or just doing ok. The fact that he's in Juvie also makes your worry totally understandable, it's a scary place to be and you have all the time in the world to speculate and over think what might be happening to him. All I can say is try not to spend too much of your time worrying about him because all worrying does is stress you out, try find a distraction, a hobby or a sport. As long as you're there for him to support him and help him along with his life then you're doing a damn fine job of being his friend.

Based on the way you described your relationship with him (feeling like siblings), I'd say it's definitely a normal reaction.

Once he's finished with this stint in juvie, you might want to try convincing him to go to some sort of therapy or talk with a professional about any anger and impulse problems he's facing. I obviously don't know him, so I don't know how open he would be to that type of thing, but the constant fighting and home issues might be able to be improved to some degree in therapy - or it might at least help heal some past issues.

I know that most people aren't super excited to go see a therapist and talk about their feelings or "problems" with a stranger, but it could be really beneficial if you're able to convince him. Hopefully this most recent trip to juvie will be a wake up call for him, whether he goes to therapy or not, and he'll realize that he can't afford to repeat this mistake - especially since sentencing is increased for repeat offenders and he's likely to be tried as an adult in the future.

The fact that the intense worry is mutual (he had a bit of an overreaction about one missed text) and he has some "home issues" might mean that you're his only (or close to only) support system. It's definitely good that he has you to support him, but it may help him to have some other people (caring friends like you, therapists, school counselors, teachers, coaches, youth pastors, etc.) that can comfort, support and give him advice (especially if you're unavailable for some reason). A therapist would be a good person for this, but it could be almost anyone who is caring, can provide rational and helpful advice, and is someone he feels he can trust - schools and churches often have specific people who are experienced in counseling teenagers about their problems or might have other resources available (like support groups).

I'm definitely not trying to add any more stress to your situation, but I'd encourage you to have a talk with him once he gets out. If you can't meet in person, Skyping is a good way to have a face-to-face talk. Just let him know how much he means to you and that you're worried about his future. Be truthful about the way his behaviour is affecting you (and him). Hopefully he'll listen to you and either make a personal decision to change or seek some help if he's having trouble regulating his emotions and responses.

As for right now, you could do a little research into what free resources might be available to him through his local school, churches or other community centers. Once you've done some research on it, go do something else. Seriously, find something to distract yourself. Go be with other people or do something that is very engaging and requires a lot of your focus. Also, I'm not sure what your (or his) spiritual beliefs are, but you could definitely pray for his safety and his future choices. Meditation (spiritual or otherwise) could help to calm you down as well.
If you call or text each other again, reassure him that you're okay and everything will be fine - as long as he doesn't get into any trouble in juvie - but try not to get too anxious... a week or two in juvie really isn't that bad, but it might be just bad enough for him to change his behaviour.

Best of luck to you both.

Yes it's normal to worry about someone you care about.

He likes you. That's what i think.