I took some friends from Australia and Sweden to the Albright Knox a few years back, and they really liked it and they were very impressed.
Take a downtown architecture tour and definitely hit the Darwin-Martin House. Do some research and pick a few buildings downtown and show him those. City Hall, the parkways around the city and Delaware. You need to be able to explain that the Guarantee building was one of the first skyscrapers in the world and that Kleinhans is acoustically perfect and is studied by people from all over. Know your architects. Drive by the new Bioinformatics and Hauptman center to show that new acclaimed buildingss are going up even today.
For restaurants, go to Park Lane, Hutch's and those types (stick with the proven winners).
Hit the Falls and maybe go up to Niagara on the Lake and check out some of the old vineyards for a day of wine tasting. If you want some adventure, hit the whirlpool rapids in one of those jet boats. I had no idea that existed and I went a few years ago and it was a blast.
I'd also hit a baseball game. The Japanese love baseball, thats a can't miss.
If he's younger and not a nerd, that opens up many more possibilities. Then you have the ballet, Chippewa and gambling that alone can make any man happy.
Good luck.