I haven't heard of a diet specifically formulated for patients with this disorder. The best one to ask would be his doctor.
If the medication isn't working, very possibly the dosage needs to be changed or a different one needs to be prescribed and only doctor can help with that. As children with ADDHD grow, their needs change and at times the diagnosis will also change.
Good communication between the doctor, teachers and family members involved with the child is the only way to properly help him. Changing dosages or his diet without consulting his doctor could do more harm than good.