Some children are willing to sit still even if there is nothing at all of interest going on. Some children seek more stimulation. Personally, I think it's a good trait in a child to be unwilling to just sit and veg out, but is always seeking interesting things to do/see.
I'm curious about your doctor warning you about this "turning into" ADHD. Certainly, with work, you can train a child to sit in a chair and fake being attentive or interested.
Honestly, sitting still is overrated.
Either a child has ADHD or another condition, or the child does not have these things. Early intervention will not prevent them, though of course knowing about such a condition can help a parent (and teachers) assist the child with coping and learning skills. Saying you need to do whatever, in order to prevent a (presumably normal) kid from getting ADHD does not sound like anything a doctor would really say.
Maybe your son has early signs of something or other, maybe he does not, but being unable to sit still for 20 minutes (possibly at a time of day when he is not tired or is stimulated from something that has just happened, like, right after arrival, or right after lunch) is not a sign of something bad. I'd keep my eye on the issue but not be too stressed. If this is a deal-breaker for the preschool, find another preschool and see how he does.
Hi, I am also the CL on the ADHD forum so do have some interest in this question.
First, to say, "His doctor suggested that I continue to keep him focused with coloring, puzzle, etc. to keep it from turning into ADHD? " is utter BS by the doctor. ADHD is not prevented by keeping him focused and that statement would make me have some concerns about your doctor in this area of medicine.
I would first find out if he is the youngest child in daycare. If the other kids are older then this may just be him compared to older kids - and he will be more immature.
If the other kids are his age (though, many times girls at this age do much better - so that comparison is worth checking out) - then I would suggest that the daycare let him move around or fidget during this time.
I do also worry about the daycare saying he has traits of autism. Nothing you have said would make me believe that. But, you really haven't given much of a detailed description. So kind of wonder how experienced they are?
I can send you links to traits of Autism, Sensory Processing Disorder (sensitive to loud noises, textures, etc.) and ADHD if you are interested. Let me know if you want more info. Best wishes.