My son thinks moving from 7v7 to 5v5 means they have gone backwards

I’d avoid trying to persuade him with a list of benefits. He is comparing numbers: seven players feels “older” than five, so in his head smaller means backwards. I’d acknowledge that it feels different, then bring it back to what he actually experiences. Ask him whether he is getting the ball more, having more chances to dribble, defend and shoot, or being involved in more moments. Those are things he can recognise himself. You can also frame it as a new challenge rather than a lower level: less time, less space and more responsibility because every player is involved more often. I would not tell him he is wrong for feeling disappointed about the change. Once the games start feeling competitive and enjoyable, the number on the format usually matters far less than it does in the first conversation.