If it's thin and short, probably the only way to do such a 'quick fix' would be to invest in a tail extension.
For the long term, braid and bag, invest in some good conditioners, wash every week or every other week(and condition), rebraid and bag. Maybe invest in a supplement to feed him as well; one that's for skin and coat.
I'm not good with fake tails; never used one. I had one horse that would have benefitted from one, but it was illegal at the time.
For long-term tail care, mom2pride and SilverSpur are correct. Wash, condition, braid and bag. Supplements are excellent for tail growth, too. The only thing I can add here is massaging the tail bone and using MTG. (Daily massage or scratching, whatever, stimulates bloodflow, like brushing does for the coat.) I'm doing an experiment right now on my mom's mare to see how well MTG works.
You can also put rings in the tail.
Part several strands of hair from the skirt of the tail and make loose overhand knot. About 3 inch in diameter with the long end of the hairs make a second ring intersecting the first all the way down. The entire tail is done this way and than spray the rings with hair spray. When the hair spray is dry (hour or so) carefully undo the chains and pick out the tail. It should be full and fluffy if it is to fluffy mist with water.
I got this trick from a book called Grooming to Win by Susan E. Harris published in 1977 and 1991 but it has a lot of great tricks and ideas for showing and much more.
For a quick solution, you can mix rice and water in a spray bottle, and spritz it on their mane and tail. It will make it shiny and smooth looking.
For growth, well, my sisters horse has a short tail, and she's always trying to get it to grow! So she uses MTG, and bandages it up for at least a week. It seems to have helped her, and she also applies it on the mane. MTG does kind of stink like wood chips or something! :P