Where to start.... First of all, lots of speech in a 2.5 year old is just that, lots of speech. Many, many children who don't speak much, or indeed don't speak at all, are very bright.
At home, he probably gets all your attention, and (forgive me) admiration. At nursery, he is one of many, and could probably have some issues with social skills. He won't get encouragement with every word he says, or everything he does.
I am sure you know, there are more than one form of intelligence, as we see it in adults (think super clever computer programmer) who are super bright but have problems with social skills, have few friends, feel uncomfortable in social situations. Children experience similar situations/feelings, even at a young age. This is why most nurseries focus on learning through play, focus on social skills, making friends, playing together, etc.
By the way, my son didn't say a word before he was three years old and now at 6, he is on the G&T list for maths and the school is considering also putting him on the G&T list for reading/spelling. He is in year 2 and goes to year 5 for maths and can easily keep up with the children there.