So I have a 200/400 runner who runs 24.54 and 54.15 in the 200 and 400 respectively. This year he is planning on running the 800 and 400 due to a dearth of 800 runners on our team this year. He seems to have speed but is lacking in endurance, last week we ran a 20x100 meter workout, as an assessment with 45 sec of rest in between each rep, he kept up with the other distance kids fine, they were only clocking around 14-15 seconds per 100, with the occasional 13. However, at the end he was significantly more tired than the other runners, and his performance throughout the week was hindered.
The next day the others did a 45 minute fartlek run, and he couldn't participate due to an illness, partially exacerbated by the workout the previous day.
This winter working out with the indoor team, he completed training for short sprints, i.e. weightlifting, 60m shooters etc. and only made the decision to convert recently, so he hasn't had time to get any real endurance building in.
My question is, what are some workouts that we could run in order to get his endurance up in a short amount of time, while still allowing him to work with the other runners, who have had base building period, and have the workouts be equally beneficial to both?