Person claiming to not have recieved gift

No shit.

Tell him to check his spam folder, or just send him the key over Steam.

He seems to have gotten because it had said gift recieved until recently so I think either he changed it to not recieved or theres been a mistake with something

I think he did Gift Recieved and then when the option came up (after 1 week) he marked Gift not Recieved just for lolz.

Contact support, they can sort this out.

Please,close this thread.

If there any way to check if he has this DLC in his Steam profile? (I don't see where it's displayed).

If it's not a key but a steam inventory gift you can check it by going to your steam inventory and click on "view inventory history" in the top right corner to proof that you gave him the gift.