IF this is his first time round penning, you must decide what your goal is. Is it have him go around x number of times? Or go a certain speed or way of going? OR is it "move away from pressure". That would be my goal.
If the horse doesn't know this, then that would be where you would want to break things down.
You want to have him move one foot away from you when you apply first body, then whip or flag or twirling a rope. When he does, you stop the pressure and turn your body away at a slight angle.
If he is eating the grass and you want his attention then you will need to have as your goal 'interupt his eating and his attention to the grass" . Do what is needed and then reward with no pressure.
It is a matter of stringing these mini goals together and making them larger little by little. But you must be focussed on what you are asking for, really askf for it with your body, your mental intention and your aids AND be very precise about releasing the pressure when you get that goal, however small.
You can also do a lot of work with the horse moving off of pressure on a long lead line, just having him back up, move one foot to the side, come toward you, stop, go over a ground pole, and on and on.
Don't let him eat the grass until he is released from working with you. But make your sessions short and purposeful.