Can my son join the Air National Guard between his junior and senior year, and then go full time Air Force when he graduates?

The Air National Guard does not have split training option, but you can join after your junior year of high school, because the average wait time for school is so long.
That said, you cannot transfer to another component until training is complete.
If he graduates high school and is still waiting for his tech school date, he will still have to finish that before he is eligible to apply for a conditional release to be allowed to enlist in the active component. And the wait for a conditional release is 3-6 months.
He could be looking at getting to active duty about 18 months after high school on that path.
If the goal is Active Duty Air Force as soon as possible after high school graduation, then enlisting in the Air Force right after high school is much quicker.

Getting from reserve to active isn't easy. If wanting active duty just join active duty.

I know you asked about the Air Force however I know that the army takes high school kids their junior year (with parental permission of course) and they get paid until they graduate for contracting. They can go to BCT the summer before their senior year and then after graduating high school they will attend AIT.