Tink Fan, if I were you, I would sit down with him and have him break this down for each class.
How does the teacher collect the homework?
-If he/she asks for it verbally, does he hear this? If no, why not? If yes, what does he do next/instead?
If students are responsible for turning homework in on their own (putting it on the teacher's desk or passing it forward)
-Does he notice when other students are turning in their homework? If no, why not? If yes, what does he do next/instead?
Like PPs have mentioned, I'd also talk to him about making this a part of his routine and strategize about how he can remember to do this. For example, saying to himself "turn in your homework" or "did you turn in your homework" every time he walks in or out of the doorway of the classroom, or putting a reminder note or a sticker or a symbol that will trigger him to think about his homework where he'll see it at the start of every class (like on a notebook).