Build a chute. That way he can have a proper approach and landing and will go forward. You can build this out of jump standard and have jumps set up to create the wall, or you can set up standards and string something like flagging, bunting, or lunge lines. It needs to be long enough that they are set up for the fence, straight to it, and have time to see it. I like it to extend the length of the long side, ending ~15m away from the short side.
When you free lunge, it's not just chasing them in circles or letting them run at the fence. Your whip and body are still aids. If they are rushing, you tell them to woah, or easy. If they need more pace, you raise the whip. You give them a moment to rest when they do the chute well. Change directions if the chute allows(grids will often increase distances progressively, so you can't reverse them). Should be very low stress, low excitement.
Always send them through the chute both directions without any jumps set up, or everything as ground poles (especially when they are new to it, an experience horse doesn't need to, but will need a warm up still). You want them to get familiar with the chute, going through it, and so their first experience isn't ,'omg a dead end, how do I get out'. Sometimes they will try to go under/around the walls of the chute if they are feeling over faced. I've had one who went between the wall and the jump wing...
Placement poles are good to help them find their pace, especially since you have minimal influence over them on the ground. Depending on what the horse needs, there are different configurations of single fences, bounces, lines, oxers, singles, ect. Usually, beyond first introductions, single fences aren't used as they don't teach them much or set them up well in the chute.
These are decent videos