I thought the same. Maybe he didn't forget but since he remembered how bad he felt with his parents ( after he used his voice) maybe he thought it would be for the best to give him some alone time. Maybe he went on the trip that day hopping he would be there so while the he is worried he said I love you cause of his voice, that's him actually saying it.
I also think he maybe somehow still remember... When he used his power to his parents they tend to fight all over again.. Since his power is "to suggest" maybe it doesn't have all the power to change one's will.. In this case he can't forcibly let Blondie forget everything about their relationship.. This maybe an opportunity to clear things out...
Well my theory but well for the sake of the story I might as well...
I wonder if it worked on Kazushi? Or maybe Kazushi only forgot the days on the beach, but still has echoes of their friendship elsewhere. I think maybe he didn't forget though. Maybe the power didn't work on him and he is here to find out why...