i think it would happen right after the second mission and just before the third one.
whiplash is still honeymoon crush period… with ax showing his hyung side and always being there to take care of sangwon and paying attention to his every change in feelings / emotions / mood and sw would notice this and reciprocate his clinginess and all of it but he’d still be a bit more cautious with his romantic feelings as it would be the first time being interested in a younger guy…
then we have the second mission where they’re separated (i really do think they spent time together then too cause sw singing ax’ line when asked to sing breathe… rlly sus) and this would instill some fear in ax which would then push him to finally confess first (!!!) after the end of the second mission. his would be a really heartfelt confession, he’d be completely open about every single one of his feelings and he’d have this glint in eyes, a mix of love, a bit of fear and his eager side showing too.
this would just set everyone in stone for sw: every doubt of his would be cleared and he’d just succumb to his feelings and accept all the love ax would give him