Translation:He's not buying it.
I think if you put a 在 in there it would change the meaning to emphasize what he's currently doing. It would be something like: "He's not buying right now (maybe he's busy and will buy later)" whereas the sentence without the 在 is more like: "He's not buying (he's not interested)"
Please elaborate. I find the simplified characters with that component particularly confusing, partly because they took the 貝 component which is normally simplified to 贝, and seems integral to the meaning of buy and sell, and replaced with 头 which is unrelated through either sound or meaning.
In the right context, it could be. If you want to specify (or emphasize) that he is not buying anything at all, you could write 他不买什么东西。Yes, 什么 also means "what?" and 东西 also means "east and west" (东 is "east" and 西 is "west"), but together, 什么东西 can mean "anything," "something," "whatever," that sort of thing, e.g.,
你想做什么东西? Do you want to do anything?
她不吃什么东西。 She is not eating anything.
他不知道什么东西。 He does not know anything. (He knows nothing.)
It won't accept: "He does not want to buy".
I see no 这 or 那 here or anything that suggests "it" in this question where in everything before there is one of the two characters above used.
Another pointless trick question where you are obliged to guess and ultimately get wrong for no real purpose.
Sometimes English responses are literal and exact, other times they are vague.