It's killing aliens yes, but it's a shared world. No bad language possible as there is very little in game communication unless you choose to join a fireteam and have a mic. If he never joins a fireteam, you need to set it so no one can join you or only plays with his mates it's fine. One of the biggest gripes about destiny is the lack of communication, which works fine for me as a solo gamer and for a child.
Part of the game is a raid, you have to do this raid to get armour to take you to level 30. This takes a team of six people working together an hour or two to complete. However..... It's not necessary at all.
Destiny is fun, and engrossing. There are several currency types with different ways to get it, and spend it on. You chose a class and level up through the story and doing bounties and things like strikes. Honestly you can have fun just doing a bounty like melee kill 20 enemies, collect a certain amount of items by killing enemies. There are public events like defend the warsat- stay next to a satellite transmitting data so 100% is sent, fighting off enemies. Several other players could be there defending too, but there is no communication whatsoever!
All in all it's fairly tame, yes you are killing but the aliens look nothing like humans, no gore or kill-cams, the bounties change everyday so it's fun even if you don't hit level 30. I have a level 29 hunter, and level 28 warlock and Titan. If even one friend has it and can play too, even if you don't talk in-game it makes it more fun.
Oh and so you know- this is an online game only. You have to be online to play, it's all on servers.