I was kindly pointed at this thread by Lieu. I spent several months in Koh Phi Phi in 2005, helping out after the tsunami. I also spent time in Phuket and Krabi, and have returned several times since.
Best recommendation is to get him to read Culture Shock: Thailand before he goes. Will demystify a lot of what’s going on, and help him from making too many mistakes.
Manwich has said a lot of good stuff. I guess if your son is training that hard, though, he may need to stay in the local area, but still explore.
In Phuket I only really know Patong (a dive) and Phuket Town (pleasant but quiet) so can’t really make too many recommendations there.
I would, of course, recommend Phi Phi as it’s stunning. There are lots of outdoor activities, and good nightlife. Tell him to avoid the muay thai challenge in Reggae Bar though, as a lot of the fights are staged, and if they’re not, some of the fighters are hepped up on yabba, and very scary (one of them threatened to kill me, and he meant it).
From Phuket it’s a couple of hours to Ao Nang by road, where Railay is. Very beautiful indeed, and set up for tourists. Krabi town is quite pleasant - bustling little place not really set up for tourists (which I prefer).
Also, south of Krabi town, is Koh Lanta, a laid-back, not-so-stunning beachy place, but with a great atmosphere and great views of Phi Phi over the water.
Further afield, it’s a few hours in a bus south to the Malaysian border, and from there you can get either to Lankawi or the Similan islands.