kp2001 is pretty spot on.
There's lots of weird starts and stops with flight school. As your son probably has told you, the pool (the water one, not the "group of bored Ensigns" one) at API was down for repairs for several months, causing people to get behind. Currently the weather at Whiting isn't great, so guys getting sent there are twiddling their thumbs and (at best) doing sims because they can't fly. Guys at Corpus are getting through the syllabus quickly, but that could change.
One of my friends reported to flight school at the end of July and did IFS almost right away but didn't start API until January. She was basically all but guaranteed follow-on orders to Whiting for Primary but (surprise!) those got switched to Corpus at the last second due to the backup in Whiting.
As for selection and all that...yeah, ask your son. But it's all "needs of the Navy." Some selecting weeks there's no tailhook spots, sometimes there's a ton, sometimes almost everyone gets helos, etc.