Excuse my language in this reply, do not read if you get easily offended.
I can't see anything wrong here. I started watching gore on the internet at age 12, and today at age 24 I still visit the same web pages as I did back then.
Gore is truly fascinating to watch. Interests could be anything from watching suicides to brutal torture and beheading. Or just simple accidents, like cars or at work.
Think about it, every single video or picture taken of a dead/dying or injured person/s in a public place always have a massive crowd of curious human beings standing around taking pictures etc.
Death is truly natural for all of us, we just aren't as used to experience it as much as they did back in the ol' days where people would die all the time. Society shield us from it, yet there's no reason to do so.
Every now and then I will come over animal abuse videos or kids being murdered etc, and of course I feel empathy towards them.
I've stopped reading news articles as I find them very A4, or media propaganda. I'd rather watch what is happening in the real world without just reading/listening to one or multiple peoples opinions. I normally disagree with them anyway. People are sheep, you know. Societies foundational bricks making life better for higher class beings.
You should be worried if he specifically target dead naked people, or dead people wearing attractive clothes. Or perhaps watching sexual torture. As this could be a trigger to necrophilia and real life situations where he wants to live out some of his fantasies. I know I do, but I still know right from wrong.
Other than that I'm a "normal" person. I have high educational skills, a very hefty work resume. some form of OCD, poor social skills, chronic depression since early childhood, self harming and suicide attempts; But I would not blame any of that on the gore as that all started way before I ever saw a guy getting his face smashed in with a hammer.
I also have gone through a lot of the phases I keep reading about her; like sniffing your siblings dirty underwear, wanting to sexually explore with my older sister, though the idea of incest intrigues me it's not something I would ever go through with. I actually don't get turned on by random sex, and can only "get it up" if I'm in real love. I also can't stand to watch porn, unless it's homemade and uploaded by people who enjoyed taking part in it.
Now you can compare me against your brother, even if you where to say that gore played a big part in who I am today, you will see that your brother is actually very normal.
People tend to really like me once they get to know me properly instead of judging me by my "restless ***** face".
I would not say this is abnormal at all. I am 15 years old as well and gore entertains me a great deal because I am passionate about medicine and medical careers. It's a constant occurrence for me to run into gory pictures or videos on the internet, ranging from removal of ingrown toenails to dead brain matter removal, and I am almost always interested and take a close look. It is a good learning experience and much better than other things he could be doing. Here is a scenario where you could say "boys will be boys".
I would say maybe he watches gore because is truly entertaining to him, I assume he does not have the sense of really or does not fully understand what is happening in those videos, these days kids are exposed to really dark material but I do not think they may see this images as adults do, they contemplate these images as a fantasy, I would say he might just grow it out, but explain to him what this images really mean, and that that is real people suffering.
I don't think theres much to be concerned about because the internet shows so many things that people consider as entertainment. However, although it may seem disturbing, it opens peoples eyes up, its what news and the media don't show so we arent exploited to content like that so we dont care much. once you watch gore and stuff like that you become interested in it because it's not what we see everyday, but it happens all around the world. I was younger too when I started watching gore too, now at 18 I'm more aware of potential dangers, but gore is still an entertainment source for me. Although as weird as it sounds it makes me appreciate life more because if he's a good and doesn't cause any trouble then chances are he's just watching them as a weird entertainment source like I did which opens your eyes up, but if he shows signs of being different or searching how to kill someone or something which gore shows to put into practise in real life then you should be concerned.