hamstring strain...

I have an athlete just getting back into comp after about 3 years off (post-collegiate). He’s been training since Jan. and has just started to run meets recently. His strength levels are better than college, he’s only about 3-5 lbs heavier, not taking creatine. Last week ran 10.61 after slowing waaayyy down at about 70m when he felt his hamstring grab. This week DNF, but the opposite hamstring grabbed this time. His flexibility is OK at best, but he’s been running fast in practice with NO problems (8.3 to 8.5 HT 80’s). Any ideas as to causes? Warm up is a/b 1 hr +, same at practice as in meets. The only thing he’s changed in the last couple of weeks is that he ran out of his vit/min supp and hasn’t gotten more. Any ideas appreciated…

Oh - and I guess I should include:
Ran the following weekend b/c the strain 2 days post seemed very slight to non-existent. Ran 3 x 80m in practice that Wed. at 8.5, 8.5, 8.3; so he seemed ready to go.
And the strain is mid- to low hamstring, no where near the glute tie-in.
Won’t run again till the 10th (2 weeks btwn), assuming all goes well, is this too soon?