Should I help out my brother or give him tough love?

My parents got a phone call from the high school that my brother is doing bad in a class; actually they weren't being very clear but they were really mad. He goes to this magnet school and is set to go to Rutgers but I'm getting freaked out he might not even graduate. From what he tells me, he says he is doing fine in class but not good in two classes which might just be ok but I'm still worried Rutgers might retract his admission. He's such a procastinator (I was and am too; just graduated college) but I actually got my **** done. I've always been on his side because his program is hard but I also feel like he doesn't give a **** and I should just let him get on trouble. Prom is coming up and he made a grand gesture in asking this girl out i helped with. I'm thinking of just offering to pick up his tux but at the same time I don't know if I should cover for him anymore in general. I don't want to condone this behavior. I'm worried that I'm being a bad sister.