Who is he a purple belt under? Huge difference between a purple belt under joe schmoe and a Caesar Gracie purple belt or something like that.
You best bet would be to practice sprawling as much as you can. He can't use his JJ if he can't get you on the ground. If he does get you down, just stay relaxed and stay in your guard if you can and try to ride it out. He's probably not going to try and submit you from your guard but rather try to pass it. As long as you maintain guard, you should be able to get a stand-up.
Mix it up a lot with your strikes, punching, knees, kicks, ect so he can't start anticipating your movements. Don't be too tentative so that he will respect your power and not get too confident, but don't get too crazy in case you make a major mistake. Counter punching is probably a solid plan since he defense will probably be shit. If you have reach on him, use it as much as you can to frustrate him. Avoid kicking above the waist early on too, otherwise he might be able to slip you up and get you on the ground. If the fights goes long and he starts getting tired and leaves his hands down, then you could go for it.
I've only been training for about 6 months now, so take my advice with a grain of salt I guess.
From what I've seen from the guys at my gym, staying calm and relaxed before a fight is just as vital as the training leading up to it. As long as you stay calm, you'll do fine. Good luck my friend.