Are you sure there aren't any developmental red flags going on? Does he make eye contact or have any repetitive behaviours? How well does he communicate with you or peers? Could be lethargic for mineral deficiencies, but a child with no smiles and blank looks concerns me. Does his hearing seem o.k.? Can he follow some directions, or doesn't comprehend at all? Keep an eye on his expressive and receptive language abilities, and how he relates to peers, and explores the toys. Perhaps he's a terrible sleeper at night at home, but keep your eyes open.
He doesn't say one word all day, other than Mommy sometimes. He doesn't often make eye contact either. If I ask him to do something I have to ask him three or four times.
I would still tend to lean to a low iron or thyroid before the autism/developmental delay route. If he is falling asleep and lethargic it would more likely be the iron/thyroid which would greatly affect his ability to interact and speak and engage. Children with autism and developmental delays, in my experience, tend to be difficult sleepers and wouldn't usually just doze off.
I have low thyroid myself and it does put you in a zombie state where you just feel like your body is made of lead and the energy it takes to follow through with a request can be a lot. So...imagine that on a 2 yr old..in a new environment and what you describe behavior wise makes sense to me.
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!!