I have had good success with young 400m runners by having them run a 600m time trial once a week in addition to their other training. The first week just run it at a strong steady pace, then each week try to improve the time a bit at a time. Over a period of about 12 weeks I have had them drop the 600m time over 10 seconds and their 400 improved by them being able to continue to run hard after 250m. You should find that after a few shots at the 600 you will view it as a challenge and then begin attacking it. Your 100m time is good enough to run below 53 for 400, but you may eventually find you are better suited for a racing distance longer than 400m, but you are young and should get faster over 100m with maturity. I have 45 years coaching experience and have had some very good 400m guys along the way.