I want my boyfriend to move in?

OK. well here's the deal. i am going to be 18 in a few months and i'm currently in a very meaningful relationship with a 21 year old. my father has vertebral deterioration and is one accident at work away from being paralyzed (this totally makes sense in a minute i swear). well my dad is having to go on disability in a couple of weeks because of his health and my mom is already on disability for a nervous disorder, and with just that amount of money coming in our family cant pay for everything. So my mother wants my grandmother to move in and help us with her check and i'm like NO WAY! well you may say "what's wrong with your sweet little grandmother moving in?...". well, you don't know( and if you do know then god bless you) what it's like to live in a house with a person that has paranoid schizophrenia. she drives everyone up the wall and can get violent(and even rip peoples' shirts off their bodies... if provoked). So i was thinking... my boyfriend comes over everyday, eats, takes naps, and even showers here occasionally and we wash his clothes too. he is currently living with a friend until he gets back on his feet and the living conditions are pretty bad. for example, tonight, he asked me to "borrow" one of my pillows because they don't have them there. i think that if my boyfriend could pay my mom the same kind of money my grandma would for rent then everything would be a lot better. well, the problem is that my mother and father would probably NEVER approve because, to be completely honest, they think we are like sex crazed rabbits or something. i really don't know what their problem is. i really think moving my boyfriend in would be a lot more beneficial than moving my grandma that has a violent mental condition in because they are just 1) adding more stress with her mental condition driving us all up the wall 2) just adding another person that is disabled to the house. instead of adding people that would actually benefit how the house is kept up, they are adding someone that we all have to take care of. my boyfriend is a hard worker and would do anything around the house for my parents and this would be a very good thing considering that my dad may be paralyzed in the next year. who will mow the lawn and do man work if my dad is disabled? we need someone stronger added to the household. plus, its not like we are denying my grandma a home. she has somewhere perfectly alright to live. she lives with my aunt. so... what do you think i should do and how do you guys think i should approach my parents about the matter at hand?