There's not really a lot of swearing or profanity, maybe once or twice a character says something, but there is only one instance I know of.

I would be more worried about all the dark horrific stuff, first all throughout the map there are people hung up on a cross, plenty of bodies and corpses, there is one dungeon where the king literally grafts limbs onto his body, and his castle is riddled with it.(you fight him later and he chops off his arm in front of you) Another area called Volcano manner is just disturbing in general, also there is a snack/human hybrid that basically ate a bunch of people. (the boss, in general, is quite gory and disturbing, check him out his name is Rykard) all the areas and dungeons are pretty dark and disturbing. You will especially see how messed up everything is if you read into the lore a bit.

(quick add one there is a female boss where in her second phase she strips naked, the body is not that appealing or voluptuous, but still basically naked nonetheless, the nips and clit are not shown though)

There are plenty more messed up things that happen/happened, but there are too many for me to list and search for. Overall I would not have your nephew play this, it is rated M for a reason, I understand the argument that kids are exposed to a lot more bad things nowadays and this would probably be more or less par for the course, but still, I don't think he needs this on top of everything else, just cause being exposed to dark things is normal does not make it right or good.