My hometown, too Ottawa is goooorgeous at this time of year, much nicer than January when most of it is under three feet of snow!
rheostaticsfan has given you some good ideas- Canada Day on the Hill and Bluesfest are both really fun. As for museums, I'll second all that have already been mentioned. The National Gallery is always worth a look, and the new War Museum IS fantastic- not just for the exhibitions but for the building itself. Great modern architecture, perfectly suited for its purpose. I also love the Museum of Nature, but that's a personal preference- if he's not into dinosaurs and bugs, it won't be that interesting to him!
For regular strolling/shopping/hanging out, he should check out certain neighborhoods, too. The market downtown is a given, as are neighboring areas like Sandy Hill and the Glebe. A little further out is the current "cool" place to live and go, Westboro. Lots of nice shops and some really good restaurants, too. (But I would say that, since I'm a Westboro girl born and bred
For good food, he should try:
Domus- cafe and food shop in the market, an Ottawa classic
Beckta- haven't been but consistantly tops "best restaurants in Ottawa" lists
Sweetgrass- one of my favourites downtown, centres on Aboriginal cuisine
e18ghteen- slightly pretentious, but good, resto/bar in the market
Juniper- back toward Westboro, delicious and reliable
The Wellington Gastropub- also in my neck of the woods, this place is really great (though I find it hilarious that it thinks it's a gastropub)
The Whalesbone- seriously awesome seafood
If I think of anything else, I'll add it.