Earlier this week, I went back and forth between the Salomon XMax 100, XPro 100, XMax 120, and XPro 120. As you may know, these boots have the Custom Shell, which allows the 98 last XMaxes to be molded up to 104 mm, the XPros are 100 to 106 mm. The boots will expand to fit the width of your foot up to 6 mm. I was in the XMax 100's last year in 27.5, but the boots had a defective liner and were replaced. I shell sized down to the 26.5 and feel I got a better fit as a result, with just enough length in the boot.
I am a long time instructor, so there is always the competition between a boot that is a good boot to teach in (we do a lot of slower skiing and a softer boot is better for that) and a stiffer boot for when we are skiing and want to put the hammer down.
As a first year instructor, your son will be teaching mostly beginners or children, depending on the ski area's needs. So that should be considered in his boot selection.
You say that he is large. If he is say over 200 lbs, the 120 flex is probably pretty manageable for him.
The XMax 100 fit me the best of all the boots. The XPro's felt like they would get too big too quick for me. I am 105 mm in my left forefoot, and 104 on the right. The shell expanded from 98 mm to my foot no problem, and I have a good secure heel hold.
The ideal boot fit is one that is snug everywhere and has no pressure points anywhere. If the boot is well fit, it will feel like it is maybe too tight the first 4 or 5 times you ski it, but then the liner packs in to just right. That is what you want to aim for. If the boot is comfortable in the shop, it is a given that it will shortly be too big.
Whomever the boot fitter is, they should do a shell fitting to see if he can downsize. Ideally, about 10 to 15 mm. behind the bare foot in the shell with the liner out. Hopefully there is a skilled bootfitter in your area who can optimize the fit for him.
I hope he enjoys teaching.