My daughter is same way
We can say adhd but truthfully they’ll tell you most kids have it which isn’t the case. If it isn’t getting his attention it just isn’t that’s it that’s all. They have more energy than we can imagine having he just doesn’t want to sit and I’m sure thinking of everything possible to get into because most know storytime is bed time 
Change your tone up and switch voices when reading then you won’t be able to stop reading to him
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