Wow, what an amazing story...and I feel your frustration. School systems seem so dysfunctional anymore. I volunteer at my daughter's school, and it's riddled with issues as well.
I am used to tutoring college-level students in English grammar, so I'm not an expert on how to teach children languages. However, I did raise a child and teach her to speak English
So I'll do my best. I think that you have to a) teach very practical, everyday things, and b) break it down into baby steps. For example, you can take a stuffed bear, and say "here's a bear!" Then find a box..."Here's a box!" And then..."Put the bear in the box!" So he's learned two nouns and a preposition.
You can go on from there. Once he's grasped that..."Take the bear *out of* the box!" Teach him immediate, physical things that you can demonstrate to him, because he won't get many abstract things at that age. If you can eat meals with him, there's a whole bunch of useful material to cover there. But just stop when he looks like he's getting tired. It's probably better to repeat a couple of things several times, than try to do too many things once or twice each.
I'm sure he's absorbing fast, though, and that you'll be a fantastic help to him. Those little minds are like sponges. My daughter went to first grade in France, learning French practically cold...then a few months later she was correcting MY French, which I've spoken for years. Hmmmph!