The only thing correct about "he" is someone saying it is correct. Language changes. "He" is a holdover from when it could be assumed that that the person in question was male. It became increasingly illogical over time. "He or she" is clumsy, "s/he" is awkward at best, and how do you say it? "It" implies a thing rather than a person. We could adopt words like "zir", but that's not going to happen.
Singular "they" is here to stay, it's the most natural solution. It's not a new idea, it's been used as a singular for centuries.