Your poor little guy. As a former first grade teacher (of kids who were going to school full day for the first time) I too know he will adjust to full day but it may take a while. Each year I always had a few kids who had difficutly with the longer day. I think food and sleep play a big role. I instituted snack time which was not a school wide thing for my class. I think young bodies need nourishment throughout the day. After snack time I would also do a quick energy boost whole body motion song or some type of game to break the school work and get the energy going.
Does your son have a snack time? My DD just started K this past week and I was so relieved when the teacher sent home a notice on the first day that K is doing snack this year. (they did not last year) The kids can also bring a water sports bottle. How her class is doing snack is every child and parent takes a turn making snack bags for the entire class (something nutritional and they gave us a list). On their appointed day they bring them in to share. The kids eat their snack while on centers so not alot of activity/instructional time lost.