Well done s-c, in trying circumstances.
Sometimes it’s difficult to interpret such a performance and deduce whether the last 50m was “soft” because he lacks the strength/endurance Or because he lacked the speed reserve to get through the opening 300m or so, comfortably.
My advice now would be that whatever the cause, it is too late to go for long reps/heavy endurance without the likelihood of tearing down the athlete (and his confidence) in this critical period close to your domestic championships.
In any case, the races he is now able (& obliged) to contest will provide the best kind of endurance this late in the day. His 400m race the other day will have shocked the system, but he is a superb athlete and will adapt rapidly.
To get so close to the winner was a stunning effort which will terrify his rivals because no doubt they will all know of his setback this season (and they know of his good competitive temperament and your ability to bring him to the peak when it matters!).
So if I can offer any advice, I’d work primarily on speed out to 300m, with perhaps some back-up sessions as in the past (such as, 300+60; 250+80) - done at race pace for the opening rep, with 2-minutes recovery for the back-up. For the speed, there are the ins-and-outs and some other work more traditionally associated with 100m race taper.
But I would also include one session, mid-week or earlier on a fresh day, of 250, 180, 160, 140, 120 (99%-effort on the sprints, with minimum 10 minutes recovery between reps).
That’s the typical session I always used in the Transition phase between GPP and Competition phases.
I suspect you’re missed that kind of work and, if so, I think it covers a couple of bases for your athlete and will harden up for the 150-to-300m zone of his race and improve his speed reserve and his speed-endurance tolerance at a level which won’t tear him down at all.
I’d keep him away from leg weights that afternoon, but he’ll be right to train well again the next day. Best wishes, kk