Speak with the teacher and remind her that you should be contacted each time your son misses an assignment. Ask that he be allowed to make up (or find and turn in) the assignments.
If the problem continues, call for an IEP meeting and, respectfully but firmly, remind the teacher and IEP team that they’ve agreed to contact you when your son misses an assignment. You might consider asking a parent advocate (find one at copaa.org) to attend the meeting. He or she will know which requests you can make of the school and what the school must do to comply with special-education law.