My kid is younger than yours, but my guess is that it has to do with missing social cues. They may not realize it’s “not cool” to hold your mom’s hand. My adhd/anxiety son is also very sensitive. They see the world differently and if they’re lucky, they have “safe people” who understand and nurture that. I understand why your instinct would be to protect him socially, developmentally and discourage it, but I don’t know - maybe it’s a gift and meet him where he is.