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?

First get your private pilot ground school done get his knowledge level up easy to do in high school .

No offense, but if he's in high school, odds are extremely strong that that A-10 will be long gone from the inventory when/if he completes UPT.
You need to get him ready for that as well

Rethink his thinking. The A-10 is been killed off in congress (bad Plan). If he is interested in being an Pilot. Advance classes would be a start. Classes in aviation.

Yes join Civil Air Patrol, strive to get the Mitchel award (lots of hard work and study) GET ALL A's if available join the ROTC this way he has a chance to go to the Academy or a college that has ROTC. Volunteer in the community, keep a CLEAN record. Those are for starters.

Take every math course he can get his hands on in high school. Join Junior ROTC, especially Air Force's version. Physically work out, build up his body strength and stamina. Stay away from girls!!! Just kidding about the last one.

get in a time machine the A-10's days are almost over

Math, science and Jr ROTC would be my best guess.

I would say a science/math degree, ROTC program, etc. He needs a commission first of all, and secondly, the AF will assign him where he is needed, not where he wants to go. Lastly, the A10 may not be around much longer if the AF gets its way. I think it is schedule to be replaced by 2021. He should be thinking pilot not specific aircraft.

Okay assuming the A-10 is still in service by the time he graduates UPT, he should enroll in Junior ROTC at his high school of they have a detachment. He might also look into joining the Civil Air Patrol because they have programs to help earn his Private Pilots License. Additionally have him focus on math and science courses to ensure he can earn a degree that's relevant to a STEM field.

His best bet is to get a pilots license now, get a bachelor's degree and join a guard/reserve unit that has the A-10 already.
Second best bet is to get into USAFA, the chances of getting a pilot slot are super high there. Then, do well in pilot training to track to fighters. Then do well in the fighter track to get an A-10 (if one drops). Unfortunately, it is rare that you will see an A-10 drop in a pilot training class, in mine (2008) not a single A-10 dropped, the only A-10 pilot was a reserve guy who was already guaranteed his slot.
Source: Academy grad who wanted to fly A-10s, got a pilot slot, tracked to fighters :)