Have to agree that good hair starts with good nutrition inside the horse. Then you are dealing with genetics. Sounds like he should have a good mane and tail. How wide is the hairline down his neck? A thin hairline just has less hair in it, so even gaining length, the mane will still remain thin. One of mine’s mane is 3" wide along his crest, he can get a serious Thelwell Pony look going over winter! We usually keep him roached to compete, so he cools down faster.
Something we do is use mineral oil on the tailbone to keep it from being itchy. Their tails are thick, shed water and we don’t usually bathe anyone over winter. Working the oil into skin with your hands, seems to prevent skin getting scaly and itchy. All seem to enjoy getting the oil worked inot the skin.
And I actually shorten tails for winter to keep them up out of the mud and ice. I usually cut them off at slightly below mid-cannon. Amazingly their tails immediately look thicker, darker with the trim. Hair DOES grow back! They have usually regained all their length by spring and I did not have to deal with terrible tails all winter!!
Do consider the chance that any tail or mane braids can snag on things, get pulled out! Had it happen here, though only skinny braids. So I quit braiding any hair except for showing days.
I like seeing a nice clean leg on a horse, so the feathers are clipped off. All the flopping hair adds to you needing to looking out for health issues of constantly wet skin, debris collected in hair making holes in horse, which takes extra time daily. I have 9 horses so that kind of time really adds up. Clipped legs dry fast overnight in the stalls, never needing to deal with scratches, cracked skin issues, too-wet hooves that can’t dry out.