Will he be a fast sprinter as an adult?
observations for Yes: As a kid and as a dad, in general the kids who were the fastest on the playground in grade school were still the fastest on the playground in high school.
observations for Maybe Not: Over the years I've known two families who had their little kids (multiple kids in each family) in the sprints in youth track. They traveled all over and won lots of meets. None of them were any good by high school. Whether they lacked talent, burned out, or just found other interests I don't know.
Being a big fish in the very small pond of pre-pubescent sprinting is no guarantee of future success. I honestly think that there are so few doing it, that if you put your kid in and he works hard, he'll win meets.
This kid is faster than the other kids in the small pond, so he obviously has some talent and maybe it's enough talent to be successful later on. We'll see.
But puberty changes everything. I was a below-average athlete as a kid but puberty suddenly gifted me with long springy legs and I became a D1 high- and triple-jumper. Meanwhile, although most fast kids were still fast after puberty, some really good pre-pubescent athletes got left in the dust after the T hit all the kids.
At my son's high school, three of the fastest sprinters didn't even run track until their senior years. All three were doing other sports and someone said "you're fast, you should run track". Once those kids show up, the kid who was the fastest at age 7 might not still be the fastest.