M.B.
This happens to a LOT of middle school children..... all of a sudden, school is for "social", not learning (in their eyes, anyway).
Although it is late in the year, it isn't too late to start up some new policies. Get him an assignment book and make it MANDATORY that he fill it out..... tell him that if he puts "no homework" in the square, you WILL check with the teacher. Many schools have teacher websites where you can check assignments, and some have an online gradebook where you can monitor their grades.
Tell him that in order for him to earn privileges at home (video game time, computer time, stuff like that) he has to show you his assignment book every night.
Have a specific space and time set aside for homework time.
Also, talk to the teachers (either in person... request a conference with his teachers, all at one time.... they can usually set this up) or via e-mail for suggestions from their end on how to handle this continual missing assignment problem.
Basically, tell him that since HE couldn't handle his own responsibilities, YOU will have to handle them...... and he might not like the results. We've had several moms escort them INTO the school in the morning for morning tutorials so they could get things finished.
I work in a middle school (inclusion paraprofessional) so I see this happening from a LOT of kids, especially as it is getting closer to summer.