Ok, so me and my family had a meltdown today.. My parents never see the real side of my boyfriend. My parents are really hard to please and they give that vibe off and my boyfriend doesn't feel comfortable at my house, so as he puts it "he goes into defense mode" and is almost a different person. He's not as nice to me and is more sarcastic and comes across as arrogant, or as my dad puts it. And it really concerns my family, they think he's this total jerk... but he's not he's so good to me.. he just acts like that because he's uncomfortable.. also he, and this might sound weird and I know what you guys are going to say to this but he looks at other girls and he doesn't even realize that he looks so much (he has ADD so it's hard for him to focus and know when he looks at a girl) so when he's at my house sometimes he stares at my sister, (and I know he doesn't realize he's doing it) and it makes my dad think that my boyfriend is cheating on me.. which I totally know he is not, I've been through this whole thing and believe me, he's not cheating so please don't write that you think he is, it's not my question.. so I talked to my parents and said that my older sister's boyfriend used to do the same thing to me but wasn't as obvious about it, and I calmed them down explaining that my boy is really good to me.. and it ended well, they are going to have a clean slate and start over

But my boyfriend knows that I was fighting with them about him (but he doesn't know the specifics) he just knows they don't like him.. so now when he comes over he's going to be even more uncomfortable and he's going to act that way again.. I don't know what or how much to say to my boy to get him to understand that he needs to treat me the way he always does, and how do I tell him about looking at my sister?

Yikes... I guess, you could mention that your sisters old BF used to stare at you and your Dad noticed it and didn't really like that guy either. That may be a good way to make him aware that his eyeing your sis is potentially being monitored.
Also, maybe you can explain to your BF that you explained to your mom and dad that your BF is really uncomfortable around them because of the vibe they put off. Basically tell him the truth (about the new slate) and tell him to stay the hell out of defence mode.

Very simple: "you act really different around my family. be yourself and i know they'll like/love you as much as i do. and i need for you to stop staring at my sister." you don't need to flat out tell him "my parents don't like you." maybe try "they don't like your behavior... " I know being outide of comfort zones can be difficult, but if you guys are serious he needs to get over it.