I would try giving him a drink of orange juice to see if his blood sugar is low, my son at 5 would fall asleep at kindergarten, the way to kindergarten ,the way home from kindergarten , on the drs examinating table and he was tested and tested quickly but turns out was in a growing spurt and had low blood sugar so some extra snacks worked well for him !
I would prob mention it at pick up time, just let mom know that Nelson seems tired and not as active, how is he at home ? I have an almost 2 yr old boy here and he is very much the same, doesnt do the crafts ect but he eats !lol Everyone elses idea of thyroid is a probable theory as well!!