cat climbing to the roof (3th Floor!!!)

Can someone please HELP me, do you know how to stop a cat going to a 3th floor and getting stuck there for 2 days? i dont know what to do, is a young male, very friendly but very curius.
We do not know how he gets there because is no trees or stairs up to the building and he does not seem to recall that every time he does it he gets stuck.
I am desperate!!!
Thanks to everyone

I don't think it's important how he gets there, but it IS important to keep him from doing it, even if you have to keep him indoors. Is he neutered? Because if he isn't you'll have a hard time keeping him inside, but his life may depend on it. Maybe you could just set up some kind of enclosure on the balcony, or if you have a courtyard, like a really, really large kind of cage (with some protection from the sun and lots of water) and make sure he's very safe in there. At least he'll be outside and able to 'visit' other cats, but not get hit by cars, or fall off the roof, or become sick from other cats, etc. Without examining your whole building in person, it's impossible to say how he's getting up on top of course, but cats are very agile and can do amazing things!

Is this outside?