Considering that it's the latest one, I'd suggest Sun/Moon as well. Might be a little rough for a 7-year old, though, is the only issue, especially with the online component now. Even the passive online stuff can be a little tricky, as Nintendo are usually good about censoring out bad words/names, but I ran across someone "from the internet" through the game and their name was "A$$Violator"... and the $$s were regular "S"s.
Could go for the more retro ones if he doesn't mind playing the one that a lot of other kids might be playing right now. Are you friendly with other parents with kids around the same age? If they allow their kids to game, what are they playing?