I did the Disney College Program and the Professional Internship Program. Both were awesome, both were demanding, both are experiences I will treasure forever!
The link for the College Program is in a post above, but here is the link for the Professional Internships: Disney Professional Internships Home | Jobs and Careers
Essentially the difference between the two programs is on the CP, you would be working frontline jobs: Attractions, Merchandise, Lifeguard, Character Performer, Food & Beverage, etc. On a PI, you would be working more specific and possibly more behind-the-scenes types of jobs: Education, Human Relations, Engineering, Management, etc.
I worked Attractions for my CP and Education for my PI. It’s been a few years since I did it but I’d be happy to answer any questions you or your son have. There is a lot of great info online too - you may want to check out the College Program board on Disboards (gasp!! yes, I know, I’m sorry, but they do have good CP info! or at least they used to). And Disney has really improved their online presence for the CP & PI. They have a blog now which would have been a really helpful source of info when I was thinking about doing the program!: https://disneyprogramsblog.com/