No product will MAKE the hair grow. Seriously. There would be no bald men. Genetics are genetics.
To enhance what is there, you can keep the pores clean and the hair conditioned. That will give the best chance for optimum growth.
However - it appears as though he is rubbing. Sheath clean? Worming up to date? And as was mentioned, allergy?
Understandable about the genetics thing. And many people don't know a whole lot about Curlies so it's hard to research this since he's such a different breed.
I've never seen him rub on anything. He doesn't even itch his butt in the stall when we bring him inside in the evenings. Sheath is regularly cleaned. He's deworked every 10 weeks, we switch back and forth between Strongid and Ivermectin.
As for an allergy, we owned him from yearling-6 years, Never had any symptoms of allergies. Sold him, he was got for 3 years, have had him back for 3 years now. So if it was something the previous owners had, it'd be out of his system by now. He doesn't appear to have any allergies. We used to board a horse that was allergic to alfalfa and his nose would break out something fierce. He doesn't have runny eyes, any baldness anywhere else on him. He's regularly seen by the vet and farrier and is in perfect health.
Just without a tail. :)
Here are some pictures of extreme curlies. Extremes are "extremely curly" in the winter time, and shed all their hair in the summer. Mane, tail, some even shed down to the skin on their bodies in certain spots. (Typically neck, shoulders, sides and butt)
Compared to those two, at least Jasper has a few little whisps of hair that he can chase flies with. Luckily my paint mare has a thick, luscious tail that drags the ground and he stands by her in the summer so she can swat away his flies. Smart boy.