Can you trial bringing him home early and skipping nap time? I spent DD's first year at infants school picking her up at lunch, taking her home for lunch and play, and then returning her to school for the afternoon session, just so she would avoid the strictly enforced nap time. It was only possible because I wasn't working, and it was a massive pain in the neck, but it was better than ruining her first experience of school, and ruining the quality of her overnight sleep.