An appy I rode once had a beautiful thick tail. Neither I, nor his owners did anything to his tail to make it this way. Another one on the same property (and actually his half brother) had a thin, shortish tail. His rider kept his tail braided and bagged, as well at putting on MTG a few days a week. The tail maintenance did help quite a bit, but he was just more prone to breakage. Though he developed almost an "average" looking tail, it was never a full beautiful tail like his brother. This happens in other breeds too (I know a warmblood with a short thin tail), but it tends to be more common in appaloosas.
Tips for growing a tail? Don't brush it. Rather wash and condition it once a week and "comb" through it with you fingers while it's all conditioned up. Keep it it braided or bagged. Make sure he is well fly sprayed when the flies come out. Keep up on his deworming. I've had success with MTG. You could consider giving him a "hoof and hair" supplement. You could do all this and you should see results, but don't expect any miracles, and it is highly unlikely that he will ever get a big, broom tail.