Lack of speed?
He ran in 5 track meets in 2016 and did not run this year. As a sophomore he did rather well for 400m. Each meet he ran in he was progressively faster. He was in that speed/growth development phase and no doubt much faster now.
Demitrius Ivey (10th).... 3rd-place......400m.... 51.78 PR..... Apr 19,2016...... FHSAA 4A District 16
He also finished 5th in the FHSAA 4A Region 4 meet with a triple jump of over 43 feet that same year. All of those that were ahead of him were seniors and juniors. Many sophomores who jump 43'+ end up meeting the national HS elite standard of 45'. The triple jump shows explosion.
Triple jump was the 19th best in the state of Florida for sophomores
For comparison, Richard Sherman (Dominguez, Compton, CA) jumped 50’8” in 2006. He ran 4.56 at NFL combine.
Also, 51.78 doesn’t flash burner, to me. But at the high school level a 400m tells you absolutely nothing about 40 yd speed.
Nothing about the numbers you’ve given prove one way or another how fast he could be. But, what I’d guess right now is that he’s not sub-4.6. Probably 4.7-4.8.