I agree with what JJ said in that it sounds like many of your problems can be solved with proper race distribution. I'd also add that 300m runs, regardless of how fast, probably won't do much to help your guy. Running 300m will build endurance for running at ~90-95% but it won't be the most effective way to build speed endurance for 100% (max velocity) running. Also, as Quick hinted at, increasing max speed will help your athlete in a couple ways:
[*]The faster your athlete becomes, the longer he will take to accelerate to his top end speed and thus have a shorter period of time in deceleration.
[*]The faster an athlete is, the faster they will be decelerating from and the faster they will be moving after deceleration. For example, athlete A accelerates to a top end speed of 11m/s and athlete B accelerates to a top end speed of 10.5m/s. If athlete A decelerates 6% (that's a LOT of deceleration) and athlete B decelerates only 3% (this is about normal), Athlete A will still be moving faster (10.33 m/s vs. 10.19 m/s) despite decelerating twice as much.