Experienced Coach wrote:
Don't cut his mileage just have him jog his runs very very slow everyday until he gets his legs back. Can't emphasize enough that the runs should be slow. He'll be fine in a week or two.
Was going to say this.
I don´t think he´s suffering from overtraining symptoms anymore, he´s just out of shape. The 16:30 5k was too much for his capacity right now, and needs to build it back by plenty of EASY running, including long runs. A really slow long run preferably on soft surfaces is very good to get the pop in the legs back soon. He can race after a couple of weeks but focus on the basic endurance runs and only workouts he can handle among with racing is some progression-LT runs. Alactic speed work and strides can be included. And when racing, aim for even pace or negative splits. No pushing too hard.