How do i tell my parent?

My boyfriend of 3 years is in jail sad thing is that he was incriminated by his cousin and i believe he doesn't belong there but my family won't even give it a double glance. Am 17 and my boyfriend is 2 years older then me and now am worried that if i don't talk to my parents they'll press charges on him for being with a minor. I've told him it's best we back off a bit till next year cause i would die if he had to stay longer because of me but to him thats not an option. we really don't want to brake up but my family won't even support my relationship and to top it all off he told me that for my 18th birthday he would like to propose i think if we announce that to my family they will seriously kill me and won't even attend our wedding in June 24 we make 4 years by the time am 18 am going to be with him for 5 years!!! how do i talk to my parents so they understand that i want to be with him and i don't want any one else.