Genetics determine the final length and thickness of the tail, but there are a few things you can do to help achieve that maximum.
Start by evaluating his diet. Make sure he's getting adequate amounts of all the essential amino acids, vitamins and minerals. There are many nutritional factors for hair growth. Fatty acids help, too.
Depending on his living situation, there are additional steps you can take. Use plenty of EFFECTIVE fly spray. If you have the time to do it right and check it a couple times a day, you can bag it. You do need to wash and condition it first, followed by detangling. Don't use a comb or brush, because you will break the hairs. Use a good product (my favorite is Healthy Haircare Moisturizer) and your fingers to separate the strands of hair. Then you just braid and bag, or bag and braid, depending on which type of tail bag you use. Be prepared to see some hair come out, but it shouldn't be as bad after the first time. Take it out every week or two, condition and reapply. Examine his stall/paddock/pasture for things that could catch the bag, or his tail if choose to not use the bag.
From that picture, the tail doesn't look too bad, but it is hard to see.
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