It is hard to type a training program out for those two events, but I do coach people like this.
I would get in a long run still, but keep it moderate. Somewhere between 8-10 miles peak. Maybe 5-6 on the low end if he has never progressed to 8-10 before.
I would then be sure to include one session of tempo runs or cruise intervals weekly. This will handle the lactate threshold. As the season progress to midseason, begin taking out the tempo run and alternate a VO2Max workout like 4x1200m@3200 pace w/ 4:00 rest in between OR the 10x400 you mentioned. Keep the cruise intervals in every other week.
This leads you to race specific work for the 400. I am big on what I call "tempo 200s" - a high volume workout that is low on intensity (about 1200 pace to start with) that progresses in intensity as volume decreases throughout the year.
You could start out at 8x200@1200 pace w/ 2:00 RI. Then add two each week until you get to 16. This will be your peak volume. Some kids only get up to 12 or so depending on their ability and length of cycle.
I then begin dropping by 4 each week, but I cut 1-2 seconds with each "drop". So if 1200 pace was 3:30 (35s 200 equivalent), then we would do 12x200@33, 8x200@31, 6x200@29, and 2x3x200@27ish in consecutive weeks. FYI - Put 5-10 minutes in between those sets on the 2x3x200. Each of those workouts in this paragraph are in successive weeks - somewhere between 7-10 days apart.
Then your sprint coach would need to come up with some acceleration work and some max speed work.
It is really tough to do.
Another approach is train them like a 3200 runner early, an 800/1600 mid, and a 400 runner late.
I've used both and been successful with it.