Wow, 70+ days a year! Amazing.
The Burton Moto is a soft boot and with that much riding I suspect he needs new boots. Calluses form due to pressure points or friction. If it's from friction usually blisters form before calluses. Do the boots look like they are falling apart? Maybe pull the liner out of the boot and inspect it for excessive wear.
The other issue you might face is that your son may soon have a growth spurt from puberty and might need entirely new gear after a year. But since you go out 70+ days a year, I'm guessing money isn't too much of a concern, also you can sell the gear he outgrows.
As for bindings I recommend Burton Cartels EST. I have their 2012 non-EST model and it's a solid binding. It's med/stiff, comfortable, and kind of like a do-it-all "middle of the road" binding. I think it would compliment the Flying V nicely. That being said, Union makes decent bindings too that have conversion kits compatible with the channel board system. Not sure if other binding companies have conversion kits. But a little research will tell you. There are a lot of great bindings out there.
If you decide to get the Cartels there are a couple gripes I have have about the 2012 Cartels: it likes to chew up the Achilles area of the boot. Burton claims they solved this problem in the newer model Cartels. I fixed my problem by applying Freesole Urethane Shoe repair to that area of the boots. The other gripe I have is that the odd time I land a jump off a natural feature the toe strap loosens a tiny bit and slips slightly out of position. It doesn't happen all the time but I do notice it from time to time when I'm unstrapping at the bottom of the run.