a common problem in green horses. They have no idea what you want from them in there! There are two scenarios here though, if he is truly afraid of being in there put a little hay or a few treats in the middle. I used to let my gelding 'weed' the round pen before we started working. (it was newly placed with little footing and lots of green).
The second idea is that he really just doesn't have any idea what you want him to do there. This is common especially green-on-green (my case as well). The biggest thing I learned was how important my body language is, and I also bought a lunge whip. Most videos told me to put pressure on his shoulder, but that just turned him into me. So I worked with it and realized that I needed to focus my pressure almost behind him and it worked!
Basically you have to find out what works for your horse. Maybe have a friend who has lunged before some out and watch you, or have them try it and see how it works. When round penning two things are so important, relationship and body language. Relationship takes time, but body language will help with that.