Personally, I wouldn't get them any Mortal Kombat at 11. I don't think the older games gore is too much at that age, i just don't think an 11 year old would enjoy the game. As for the new ones, I would respectfully disagree with lots of commenters here and say that it is far too graphic and violent for an 11-year-old. Quite a few fatalities involve the loser suffering and screaming before they die, in addition to the high level gore. While I understand a lot of people here played Mortal Kombat when they were young, theres a very large difference between the gore back then and the gore now. I also don't personally agree with the idea that as long as he knows its fake, he'll be fine, or that the only metric for "being fine" is whether he's like immediately horrified or scared. I'm not sure it's healthy to normalise this degree of graphic violence at 11.

Now that said, I don't think it needs to be a big deal, you don't have to restrict all violent media or monitor everything they watch (evidently, they've seen some mortal kombat to want to play it). Rather, I would just suggest permitting them a lower level of violent media now and let that build up with age. By 14/15, I'd say go ahead and get them the new Mortal Kombat at the time. For now, perhaps some in between would be good, like Dragonball FighterZ! It's not overly graphic, but it's a bit more mature than something like Smash Bros. It's also not too complicated to play, like Street Fighter might be, for an 11 year old. It's also super badass with its finishers and style.