Hi Anistark, because you posted this month, you are also entered into this month's drawing. Good Luck!
How large is his playpen right now?
I do agree that confinement is a good idea, especially since he may begin displaying territorial marking soon since he is reaching that age of sexual maturity, but depending on how big the play pen is, you could double it up with another play pen and expand it, or allow him strict supervised freedom. I do know that one of my friend's bunny did okay before he was neutered with limited freedom, but it really depends on the bunny and how territorial he is.
If you find he is marking in your house, then you'll need to prevent bad habits from cementing in and confine him back to his playpen.
Let us know what you decide and how it goes.
If you decide that confining him is best, don't worry, he'll have plenty of time to be free, and you will be happy to have a completely litter trained bunny.
But also note, that it will take up to a month after surgery for his hormones to die down, and even after that you will still need to supervise and train him to go back to his litterbox if you see him lifting his tail to urinate, and or seeing him marking with poos.