My nephew is interested in becoming an A-10 pilot in the Air Force. What should he start doing now while he's in high school?

He'll need a Bachelor's degree to be commissioned and become a pilot. Plus he'll need at least 20/20 vision uncorrected to be a pilot. Tell him to stay in school and go to college.

He first needs a college degree. He can go through ROTC or apply to the academy or just go to college on his own Then apply to flight school after he gets his commission

Lots of good comments about commissioning sources but that's at the very best only 50%. And I agree with the CAP recommendation. His first big post-commissioning milestone is just to be selected to pilot school so here's the rest:
1. Work out at least every other day and get in top physical condition - aerobic and upper body and core - lots of pushups and pullups and lateral exercises, lots. And if he can't run 5 miles, depending on commissioning source he may wish he could. Especially if the Academy or something similar is the target.
2. Protect his eyesight like it's gold, always wearing sunglasses outdoors. Always.
3. Keep grades high. Impossible to overstate this.
4. Become an expert with Microsoft Flight Simulator but again, protect your eyes. Check out Gunnar computer glasses. MS FS has developed astounding proficiency.
5. Again agree with flying lessons such as with the CAP.
6. If you wish to pursue USAFA, don't view it as a single point of entry such as through a Congressman. Many ways to get there. Myself I had no chance at the Congressional route so I went enlisted. Spent 6 months and then applied and got accepted to USAFA.

Hope that helps.
Bill

Private pilots license can help but not required as well as just any flight experience. When taking the AFOQT studying the pilot section books available just like SAT study guides.

Private pilots license can help. When in college studying for the AFOQT specifically the pilot sections.

First I would tell him to get interested in a different air frame. It's a shame but the a-10s days are numbered. After that tell him to start researching on how to get into ROTC or the AF Academy and the requirements to become a pilot.

Math. Math. Math. More math. Then, when he's done with that...MORE math.