How can I show a non-religious person that abortion is wrong?

Try this: Point out that our society is built on the principle that every innocent human being has an inalienable right to life. If we can show the unborn are human beings, then we must accord them the right to life.

Next, ask him to look at the question of the unborn's humanity from a strictly scientific perspective (no religious propositions). Scientifically, what is a human being? The answer is: a living human organism. If it is a human organism, and if it's alive, then it is a human being.

Do the unborn meet this definition? Yes. They are living human organisms. The fact that they are living is demonstrated (among other things) by their growth. The only question here is: Are these living things human?

They are; they possess a human genetic code—no other animal's or plant's code matches. So, if they are living and human, are they organisms? Yes; they are organic wholes with their own inner principle of life and development. The unborn are not just globs of cells but complete organisms. Take a glob of cells, leave it alone, and it will remain a glob of cells. Take an unborn, leave it alone, and it will turn into an adult human being. We must conclude, therefore, that the unborn are living human organisms, which means that scientifically they are human beings, and so must be accorded the same right to life as anyone else.