Battler probably knows he said bye to her and "promised" to come back, but maybe it was just a joke on his part? He was only 13 when he said it, or he was probably just so shy he said it to sound adult or something.
Battler maybe just forgot the love. George is such an ass, he maybe sent letters to Battler saying, "Oh ya, Shannon's doing great without you~
" just to sabatoge the relationship more(LOL sorry I dunno if they ever had contact, just kidding)
I hate George, if only he gave Shannon the darn letter, it would've probably changed things D:<