Can someone please help with this?

hi, im going out with my boyfriend two years. He has a history of chatting up girls behind my back on facebook id usually find out by friends telling me. So i eventually forgave him but got his pword for facebook to keep an eye on him. I did'nt catch him doing anything until one day i thought of his yahoo account wich had the same pword, when i checked this there was various msgs to girls hed been chatting up asking for there phone no.s telling them they were good looking e.t.c; So we broke up, but now after a while im back with him again and i have his fb pword but he didnt give me his yahoo account hes saying now that im been to nosey and to trust him. I ended up guessing his pword for his yahoo account but its empty and its not that hes deleting as he goes along there is literally no msgs in the account not even msgs fowrded to his account from fb. I know use are going to state the obvious and i really shouldnt be with him if i cant trust him, but all the same i really want to know how he can stop fowarding his msgs from fb to this address when its the only email linked to his fb account? or if he set up a whole new yahoo account would he be able to get all his fowarding msgs from fb with out linking the new email to his fb account or if he has to link the new address is there a way he can hide showing the new email address even to himself when he logs onto his fb page and in to his setting ?? please help im so wrecking my head over this.