How do I treat a 5 year old with autism?

It's hard to give a specific answer to your question because autism affects each person differently and a various degrees. The best thing you can do is educate yourself as much about autism as you can, the more you know about it, the more you'll understand it and can help him, a great place to get accurate information is the Autism Society of America, http://www.autism-society.org/ . You may want to get in touch with his parents/guardians, they will be able to give you the best info as to what he likes/dislikes, etc.

I do suggest reading an excerpt from a book by Ellen Notbohm titled "Ten Things Every Child With Autism Wishes You Knew", this will give some insight to basics of autism. http://graphicpieces.com/autism10thingschild.html

I taught swim lessons to a kid with autism. Very frustrating at first. It really depends on their personality. Some autistic kids are very aggressive and highly deviant, others are more subtle and nicer to deal with. Either way it is easiest to communicate with them with visuals and physical motions. Talking is not going to do much good in my experience.

Just be really nice. They act pretty much like little kids so as long as you play along with him everything should run smoothly.