HOW DO I START MY CHILD ON A GFCF DIET?

I have an 8yr old son who recently was diagnosed with PDD NOS. I have done some research and thought about starting the GFCF diet. Not sure how to start. He is extremely picky and has a severe gag reflux even looking at foods he doesn't eat makes him throw up. Any ideas of how to get him to try other foods or the best way to start this diet? I would like to start by stopping cows milk but I m not sure where to find Dari free milk these sites talk about? Any suggestions. Thanks.HOW DO I START MY CHILD ON A GFCF DIET?
I have no idea what any of those acronyms stand for but you can get soymilk, almond milk, and rice milk from any grocery store. A bit less common is a coconut based milk. Out of the four, almond milk is my favorite, but soymilk is probably the healthiest.