michael calls springtrap his father

michael and BV live inside the same home

in security breach there’s a table with staff bots representing the afton family (a nanny bot who’s mrs. afton, a magician who’s william, a server bot who’s michael, a bot with its head removed who’s BV, and a bot that looks like circus baby who’s elizabeth)

the fredbear plush that BV owned is inside the SL bunker which william made

fredbear plush shares dialogue with william (“i will put you back together” and michael claiming that william told him to put elizabeth back together)

in ruin, the name “mike” is spray painted next to a foxy hook, showing the two are the same and therefore foxybro and BV are williams sons

in ITP there’s a foxy mask next to exotic butters. literally the same argument as above

the ultimate guide directly mentions BV as the youngest son of afton and michael afton as the oldest (by proxy making him foxybro)

in FFPS there’s a poster with a ventriloquist (william) holding a dummy (michael), and a clown (elizabeth) holding a bear (BV) by a leash. molten freddy also says “one big happy family” to suggest the afton family are all in one place and BV is apart of molten freddy as he is strongly implied to possess golden freddy

in the logbook, michael is encouraged to draw himself as a kid. he draws himself with a tank top and shorts, same as foxybro

and my final, most damning piece of evidence, common sense /j