Just because he seems sound when being ridden, doesn't mean he is. Horses are prey animals, therefore very stoic.
He could be hiding an issues that isn't flat out pain but is discomfort for him, and it shows when you ask him to lunge in a certain direction.
I have seen it with my own horses, when the chiropractor sometimes lunges them to see if what she did worked.
Which, BTW, is why I made sure all my horses knew how to lunge at a young age. Except for learning some disicipline early in life, and having it come in handy when one of their professional health care people need to watch them, I think it is a waste of time.
When I used to go on organized trail rides, I could always picks the rider who was likely to go off in the creek ------- the one who spent fifteen minutes lunging her poor horse at the trailer.
If one has to do that before riding, one needs to take a step back and look at what THEY are doing wrong.
Instead, play some liberty games with your horse. For starters he is an Arab, so already has a leg up in the brain department. They are fantastic at liberty work.
Start simple. Something REALLY worthwhile is ground tying. All my horses knew/know how to ground tie, some better than others.
I have to bath my horses outside. I haven't tied a horse to bath in 20 years. At the least, I just throw the rope over the fence.
I used to tell TWH Duke, my avatar, to "let's get a bath, go find your spot". He would walk to his spot and never move u til I said "you're done", when he would turn and go back into the barn to his stall. I once made the mistake of saying "you're almost done". He heard "done" and was headed to his stall before I got the shampoo rinsed off. I never made that mistake again, lol
Meaning, it was a simple liberty trick that he learned and understood
I have taken one Doberman and two Rotterweilers thru Puppy obedience. You wouldn't believe how much if that (including hand commands), I have been able to transfer to horses over the years.
Horses all have different brainpower, like dogs, like people but they are willing learners when given the chance.
There are so many little "freedom" things you can do that are much more important than lunging. They will better stimulate the horse's mind, he will have a lot more respect for you, and you never know how those little things might come in handy in an emergency.
As the Food Lion lion says: "Just my two cents".