Do i tell him??? or leave him baffled?

So me and my boyfriend broke up a few days ago...i always had an idea he was cheating on me and his best friend finally confirmed it...this past 6 months that ive been with him hes put me through so much **** as well..hes literally brainwashed me and he has a narcissistic personality...ive never felt so stupid used and hurt...we got into a fight the night before i found out and the next day his best friend told me...hes been calling me and texting me confused cuz i completely stopped talking to him and hes wondering why i just completely ignored him out of nowhere...I dont know whether to leave him baffled and confused just like hes left me or let him know i know everything...either way even if i tell him i know hes going to deny it and i cantt confess that it was his own best friend who told me...what should i do??