Have you talked to his year level co ordinator? If you have and got nowhere then the vice principal then the principal. The school should be able to sit with you work out a homework plan to help him. Has the school a reading recovery program, if they do he should be in it. It is a great program with one on one reading help.

If he doesn't mind repeating year 7 then I would suggest he does and that he also gets an edu assistant. If they offer one for year eight then one for year seven should not be a problem.

Get him to read to his younger siblings and even make up stories for them and then maybe write them down. If he likes sport get him to read about that sport and then tell you about it. It will mean you will have to know about the sport also so you casn pick him up if need be.

Are you close enought to go visit someone from the edu board. If so make an appointment and talk to someone face to face. It is harder to be fobbed off when face to face.
The other thought is getting him tutored after school