However, this was a bit more illuminating:
Ironically though, Pokemon is pretty much the best series ever for allowing players to keep their favourite old Pokemon relevant in the newest instalments. In that sense, it's amazingly good at preserving the good old times through the new games. But what are you going to do? If he wants to move on, let him. No point in trying to urge him back. If he ever wants to, the games are there for him to play, and if not then hope he enjoys whatever he does with his game time now!