Has the teacher tried a visual schedule with him? These are often made from picture cards (line drawings) that the child has on his desk to help him follow each step for work completion. Also, does your child do better when he is moving? He may need a work station that he can stand at or a move 'n sit cushion or therapy ball at his desk to provide movement while he attempts worksheets. Sometimes kids on the spectrum really need fidgets to keep part of the body moving while the mind focuses on work.
I agree with imf64 do NOT offer to have him finish work at home - if it is that important for his learning for him to complete it, it needs to be done at school. Home should be for teaching other life skills like taking care of himself and doing chores as part of a family.
Has your son had prior support such as Special Education preschool or school based Speech Language Pathologist (SLP) or Occupational Therapist (OT) services? If not, I strongly suggest that you request an evaluation for special education due to his medical diagnosis adversely impacting his ability to participate in a free and appropriate education. Areas of concern would include not only IQ and academics, but also language, social/emotional, attention, memory, and behavioral concerns.