It's pretty unlikely that your horse is scratching himself bald over parasites, especially since you've dewormed with ivermectin. No need to just keep dosing him with different drugs every so often.
Check his sheath and penis carefully for beans, parasites and just general filth as this is a common cause of tail rubbing in male horses (in females gunk in between the teats can cause it). And check the tail itself for any sign of parasites--from attached ticks to very tiny moving things (lice) or small white things that look similar to bot eggs attached to the hairs.
Licensed Veterinary Technician