Baby Daddy in baby book

So long story short I was raped by my sons dad, he's a horrible person with multiple arrests, will lock his son upstairs to go have sex with girls, leave his son in the car to go into a party and find the drunkest girl get her drinker and rape her Etc. he's a horrible horrible person. So we got the hell out of there. I don't want my son to hate me in the future for lying to him, many people have told me to just tell him his dad is dead. But I don't think that's right. I kind of want to write a letter for him in his baby book to tell him who his father is and a little about why he's not in our lives, I just don't know HOW in detail to go with it. Or if I should just tell him he's dead, I don't know! He's only 3 months old, I wouldn't give it to him until he was old enough. Anyone have ideas or experience with this?