exactly what I was going to say - all kids (and adults) have differing attention spans and interests - nobody questions why some adults like to be on the go all the time and have active jobs whilst others are happy to sit in front of a pc all day, but sometimes they just want all kids to be the same. I guess it is easier for the nursery if he will sit quietly and listen at story time. I know that this ability will need to be encouraged and nurtured as going through school he will need to conform, but it is early days and there will be kids at school who have not yet mastered this art!
My son got into trouble at school when he was 5 and wasn't sitting quietly at school and they called me in to discuss and support them. i am generally very happy to support them, but to be honest I didn't know how they wanted me to reinforce this behaviour at home, because i dont really need him to sit on his bottom on a wooden floor for an hour lestening to people talk about stuff he isn't interested in. Anyway, 1 year down the line, he is teachers pet and gets the star pupil award regularly for his great behaviour, hard work and mature attitude. Just give your little boy time - it sounds to me like there are a lot more fun things to do aat nursery than listen to the story and when he doesn't have all those distractions he is fine.