that works for me. maybe their moms even met in hospital when they were having them? sort of "betrothed since birth" without anything binding or anything like that. now that i think of it, they could have a cradle to the grave sort of bff line with the high school sweethearts aspect thrown into the mix for a year because of pressures & curiosity?
if he moved when he was 5, we can just say they were neighbors and then the rest could still apply?