I have a friend,we met online 5 yrs back,it clicked n now we hv reached a comfort zone where we can share?

anything abt our lives-even our email pwords.he is my best friend.he shares every problem of his life wid me n supports me when i need him.he had a bad fortune in his love life(he loved a gal n it dint work out as she doesnt)n maybe that brought us more closer.6 months back he proposed me.i told him its not possible coz of some personal reasons(m not dating anyone) n we continued being frns.few days back he told me seriously that he hs crush on me.this time i gav it a thought.he is busy these days so thought we cn talk abt it later.yesterday,i asked him for some seminar topics which he gave last year.so he told me to search in his mailbox.i accidently(it wsnt done purposely)opened a mail written to his frn last year in which he talked abt date they had(not the girl he loved).i share every part of my life wid him.even he does.but he nvr evr told me abt this.he says m his best frn.when he shares evrything,evn abt his family,y he dint evn mention abt it.wat shal i talk to him abt it?