Is this your child with food allergy? Don't underestimate the impact of that fear on social anxiety in a child at preschool age. Most NT kids with food allergy go through something similar at 7-10 yo, but HG kids with food allergy seem to undergo a really different developmental experience, because they are processing mortal danger from sources that they can't avoid/control at a much younger age-- an age when most contemporaries can't really understand what that kind of risk means.
KWIM?
My DD presented as VERY socially reticent at this age-- with peers, that is, and very much more socially gregarious with adults. She, too, had a lengthy attention span (hours, if she was interested). She had a lot of the superficial markers for an ASD. But it was almost entirely HG + anxiety, and not all of the anxiety was maladaptive or irrational, frankly.
