Let me give you some good references. The fist is a great overview of autism from the Mayo Clnic.
HERE IT IS
Here is another from the National Autism foundation...
Finally, THIS LINK HERE will be helpful to you.
As you will learn from this, each autistic individual has different degrees of disability due to their syndrome. A careful, slow approach is the best bet for you as you get to know your partner's child.
Please let me know if you have further questions for me. Good luck to you sir as you move forward. It speaks well of you that you are trying to get ahead of this potentially difficult situation.