MDJ - PUT DOWN THE SCISSORS!!!! Tails are my passion! Comes from the old days with Saddlebreds.
Unless he grows a whole lot of tail per month, you won't have a show tail by next spring. I would guess, max you would get is 6-8 inches.
Wash the white tail in purple shampoo and leave it on for 5 minutes,(make sure you scrub the bone very carefully) rinse and put your cream rinse in, then wash it out. Use what ever product you like. Spray the tail with Show Sheen and gently pick out the tail until all the tangles are gone. That means take 3-5 strands of hair out at a time and ease the knots out. By the time you have your tail completely picked out, it will be close to dry.
When the tail is completely dry, loosely braid below the tail bone, leaving the top "feather" out of the braid,. I also leave a little piece of the long part on each side of the braid lose to swat flies with. I braid to the end of the tail, then wrap the end of the braid with black electricians tape. Fold the end of the tape back so you can find it when you want to take it off. (It will not leave any residue) Fold the braid up through the top of itself below the tail bone, (that's why you want to braid loosely). Wrap the entire folded braid with electricians tape or vet wrap, encapsulating it completely, but not touching the tail bone. Make sure everything is lose at the bottom of the tail bone or it will pull hair out. This needs to be taken out every few weeks and rewashed in purple shampoo to keep whitening it even more. Mind you, I only have male horses. If your mares won't hold their tail enough to the side to not pee on the bundle, you will have a problem. If the bundle gets wet and not dried out, it can mildew.
By next spring you will have more tail than now(thicker), and it should be long enough that you will want to cut off the excess at your preferred length and shape. Personally, I cut the tail off straight like the qtr horse show tails, at the ground. Some people like a more natural shape. You will lose some tail everytime you shampoo and pick out, but this is natural shedding, if you are not breaking the hairs when you pick. The tail should be nice and white be then, if you are using purple shampoo (whatever brand)
I find the trick is to never leave the tail unbraided. Each time they swat a fly, it can break hairs. I have used this method on every horse I've had. It works even on horses with not much hair. Max had a nice tail. Some of it was the silver color, and that will never be white. But the white part was very white.