I already speculated a while back that Russ has poor or no sense of smell and taste due to zinc deficiency (likely both due to autism and his terrible diet), which would affect his choice of food. However, autistic people also tend to crave carbs even if their senses of smell and taste are intact. I can't imagine that having his mouth open all the time has done his taste buds any favors either, but who knows on that one. I mean, his parents might have resorted to bribing him with sweets when he was a little shit, but there's multiple plausible biological reasons for his taste to be complete garbage without blaming his parents.