In order to answer this question, you must first understand Venn diagrams. A Venn diagram is a tool in logic that visually illustrates what is different about or the same between two or more objects, ideas, characters, or other types of concepts. Venn diagrams are represented by circles that either are parallel or overlap. If comparing two things that have no points of similarity, the circles would rest alongside each other and their boundaries would not overlap. Visually, then, the logic of dissimilarity and exclusion would be represented. If comparing two things that have points of similarities, the circles would overlap the lesser or greater degrees. Visually, then, the logic of similarity and mutual inclusivity would be represented.
You need to understand these fundamentals about Venn diagramming before you can design a Venn for David and Jason. Once you grasp the fundamentals, you can assign a circle to David and one to jason and make your lists of what characteristics David and Jason share and do not share. Afterward, you can decide whether they share characteristics to a greater degree or to a lesser degree. This degree of inclusivity of mutual characteristics will determine the amount, greater or lesser, of overlap needed for the boundaries of the Venn circles. Here are a few of the most obvious characteristics share and not shared by Jason and David.
David: NOT SHARED: no trouble with speech; his sister is his assistant; his mother is philosophical; walks. SHARED: goes to occupational therapy; has cognitive dysfunction; experiences the world through a function impaired perspective.
Jason: NOT SHARED: much trouble with speech; no sister who is his assistant; mother is anxious and not very philosophical; does not walk. SHARED: goes to occupational therapy; has cognitive dysfunction; experiences the world through a function of impaired perspective.
From this brief sketch, it appears that on a one-for-one basis, there are more things that David and Jason do not share than that they do share. So in your Venn diagram, as you fill in more detail to the NOT SHARED list, the degree of overlap of the circle boundaries will be less than a one-third overlap, certainly not as much as a one-half overlap of circles.
Yet, alternately, if you look at the lists another way as through the lens of life impact, it might be argued that the things they do share, though fewer in number, have the greater impact on the way they feel and live. So if you were to make a Venn diagram to visually represent the total impact of "SHARED: goes to occupational therapy; has cognitive dysfunction; experiences the world through a function of impaired perspective," your Venn would have deeply overlapping circle boundaries, certainly more that a one-half overlap of circles. For example, even though David speaks while Jason does not, both boys impact their worlds yet feel no effect--or cannot show that they can feel the effect--of that impact:
David has to ... walk down each row of videos, flipping boxes over to read the parental advisory and the rating -- even on videos Dad would never let him rent. ... He'll keep reaching for boxes and flipping them over, not even seeing the looks people give us.
"Girl don't? What girl?"
Everything falls quiet. I glance up.
Mrs. Morehouse is staring at me. "Are you drawing my son?"
My pencil freezes, mid-stroke.
"Just because Jason can't talk," she says, "don't assume he doesn't mind!"
Everyone looks at me. ... "I'm sorry," I whisper.