enabling/ making it within reach

My son really struggles to complete any task. He is in Special Ed and has some processing differences, and gets really overwhelmed with the smallest tasks, but has also told me that he "knows what to do to get himself out of work." Its really hard for me to know what he is and is not capable of —–and when I'm giving him too much slack and when I'm not giving him enough. For example, in "24 hours" or he might get 1/15 of the work done that his younger sister could get done in that same time frame because he is moving so slowly and getting so distracted. Is this his "A" effort and should be praised? Or his "D" effort, trying to get out of chores, and should be given consequences/ another 24 hours? What do you recommend?