My friend is 17 and functionally illiterate. How do I teach him phonics?

Pretty much what the title says, my friend is 17 years old, can’t sound out words or guess pronunciation. I coached him through sounding out some unfamiliar words and realised I am way out of my depth. As he is American and homeschooled he doesn’t have many accessible resources and I need help finding them!

He is one of the sweetest people I know and I hate that the education system has failed him so badly. I really want to help him out, he’s super eager and grateful to learn but we’re a bit lost on where to start.