It is obvious to you already that he is an advanced learner. You are honest with yourself that you're not one of those parents that hot-housed him. Instead, you're one of those in absolute shock and surprise that other 3-4 year olds don't already read.
If the school you are hoping to attend does *not* require testing, then you can hold off and wait until he is 6 where you can get better results. It is pretty obvious already. One tactic I use to help those around me -- is I act just as shocked as they when I tell them my child can read already. Really, it isn't something you can force a 3-4 year old to do.
If the school you want to attend requires *proof* you should see what kind of proof they want.
I've read on this site too also, you want something "Appropriate" for your child, in terms of educational level. Everyone can agree that there has to be something "appropriate" for him.
The testing will confirm what you know already.
If you wait or repeat/ or do different test at age 6-7, you may get more information about just how advanced he is and if there are any disabilities, etc etc
Goodluck!