Small world! My DS7 wants to learn Arabic, too.

He took French on Saturdays in a GT enrichment class from a man from Morocco who speaks Arabic, too, and he got interested.
I haven't found anything to purchase or online that I like. Luckily, my DS4 is in pre-K with the kids of a couple of native Arabic speakers. (Trust me, that is VERY unusual in our white-bread-and-mayo midwestern community!) So I just asked one of the moms about it. It turns out she's planning to teach her own son Arabic, and she said I could add him to the "class" if I wanted to.
My best suggestion based on my experience is to find someone in the community to ask about it. If you have a university or maybe a mosque nearby, you might inquire there or post something about wanting a native speaker to teach your son.
Also, I'll let you know what book our teacher is using. I know she has one picked out, but I don't know what it is yet. She did tell me that it's for writing Arabic for people who can't speak it. That's all I know.