My parents dont like my boyfriend what should i do?

My parents didn't know that i had a boyfriend and my dad found out somehow and right now my boyfriend is in jail for bodily harm. My dad found n out and now he doesn't want me to date him anymore. i have been sending my boyfriend letters and my dad told me not to do it anymore. Don't know what to do??i don't know if my doesn't approve because of age difference cause im 17 and he is 20 or because he thinks my boyfriend is a criminal. i dont know wat to do i dont want to stop writing letters to my boyfriend.. i dont want to break up with him either.. any advice??