he has to call MS to add it to the ressie but don't leave it to the very very last minute (48 hours before arrival). some "more popular" restaurants do get really busy and some people really mark their 180-day (you can make ADR 180 days before) so they can call and reserve their favorite spots. you will be competing with them. there's tons of restaurants though... i guess as long as you're not dead set on a specific one at a specific time then you'll be fine.
have fun!