Honestly, if he's a great eater, I'd keep him on rats. Really, they grow the same as long as you are feeding them an appropriate sized meal. Sometimes, if they switch to ASFs, they don't want to switch back. That's why most people only feed ASFs for a picky eater. Not to mention, if you ever need to sell him, the fact that he'll be an ASF eater can potentially be a turn-off. Since they aren't as readily available, most people don't want to own ASF eaters.
Just keep him on rats. Feed him appropriately every 5-7 days (stay at 7 days when he hits 500g or so) and he'll grow fine.
