It's not that he's "not letting his tail grow", he's most likely breaking his tail-feathers off on his cage bars, while playing, etc. If you are actually seeing him pulling-out his tail-feathers or chewing on them, then you need to get him to a Certified Avian Vet to get a wellness-exam and to have a Fecal-Culture done...Often times when they are purposely pulling=out their tail feathers or plucking the little feathers around their vents, it's because they are suffering from either a Gastrointestinal infection of some kind, or a Urinary Tract or Kidney infection of some time, especially for such a young bird. So that's not something that you can wait and see on if you're actually seeing him purposely pulling out his tail feathers. However, if you haven't seen him purposely pulling his tail feathers out or chewing on them constantly, then he's most-likely just being clumsy and knocking them out on the bars of his cage and while playing...
Also, sometimes if they are constantly knocking out or damaging their tail feathers (not purposely pulling them out or chewing on them, but doing it on the bars of the cage), it's a sign that their cage is too small for them, usually it's not wide or deep enough. So that might also be the issue.
Another thing that you need to watch is the amount of fresh fruit that you're giving him, as fruit is loaded with sugar, and should only be given as an occasional treat and in small-quantities, it should not be a part of his daily diet at all. His daily diet needs to consist of a staple, or the main-component of his daily diet needs to be a healthy, fortified pellet diet, with a small supplemental seed-mix that contains no sunflower seeds but rather a good mix of different healthy seeds, grains, legumes, dried veggies, peppers, etc., and the seed-mix part of his daily diet should only be a small fraction of his staple diet...And then he should be getting a lot of fresh vegetables every single day, he can eat as many fresh veggies as he will eat every day, the easiest way is to make him different types of "Chops", which are just different mixes of chopped-up veggies mixed together. You're best to make him chop-mixes in large batches and then put single-servings of them into little baggies and freeze them, and then you can just take them out of the freezer the night before and put them in the fridge to thaw-out....And with the exception of NEVER giving him any onions or avocados, he can pretty much eat any other fresh veggies (both onions and avocados are toxic to birds)...But as far as the fruit goes, if you give it to him every single day, please only give him a very small piece or two and that's it, because again it's loaded with sugar, and that sugar turns right into fat that gets stored in their liver, causing Fatty Liver Disease..And it can also result in them becoming diabetic. I try to only give my birds fresh fruit as a treat 3-4 times a week...