Hi ESTI
It’s hard to say without seeing him race and without more 100m segmental splits, but you are probably correct that he could get a faster 400m even based on where he’s at with his short speed (200m PB) at the moment.
There is a basic formula that you can run the first 200m in a 400m race no more than 1sec slower than your season PB for 200m. If your athlete can run 23.3 for 200m at the moment, he probably should try to go through in 24.4.
There is also a basic formula that the second 200m (back-end, come-home) in a 400m race is about 2sec slower than the first 200m. In a very well conditioned 400m racer, that can come down to around 1sec differential. But it can also blow out to as much as 3sec+ when a speed monster attacks the event without sufficient 400m conditioning or if they are just reckless in the opening 300m or so.
Anyway, if your athlete constructed his 400m opening in 24.4-ish, and if he could apply the basic 2sec differential “rule”, that would give him a model of 24.4+26.4 = 50.8 off blocks.
But there are many ways to ruin a 400m even by a supremely well conditioned 400m specialist. Taking the turns properly is key. As coach, you need to go the the infield and observe how he takes the turns. He should be sustaining Triple Extension as well as possible on every part of every bend. As a coach we focus primarily on his left (inside) leg. Look to see that angle behind the knee. You don’t want to see too much bend. They should be looking no further ahead than about 10 metres and thereby try to run the bend like it is a straight-line. That is the most efficient way of getting it done.
Also as a coach we need to get on the track and watch the athlete from behind as he eneters the turn (or exits it). Often an athlete will “drop” the left hip and sort of hunker down to accelerate the turn. We don’t want that. We need to encourage lateral stability across the hips. We don’t want any collapse on the left side (inside) lever.
Of course we also don’t want the athlete afraid of the bend, ideally we want them carrying their momentum out of the backstraight through the 200m start zone on the bend. Keep the momentum going, keep the left side tall, keep the hands/arms working (hands should reach shoulder level, but shoulders should never be “hunched”).