If a sub 10 sprinter can’t hold their top speed for more than 20m then there is no way in hell an 11 second sprinter can. He should start having a plan for how he distributes his race, especially if he is running into a headwind or if its cold.
Read Shaver’s presentation from nacactfca.org ( https://www.nacactfca.org/Shaver – Sprint Training.pdf ) around slide 80 and on he talks about race distribution for various distances including the 100.