If brand and foil type weren’t a factor, what would you say is the single easiest foil to learn on for linking waves and improving pumping?
I’d put my son at beginner/intermediate — started about two months ago but foiling 7–10 sessions a week, and main goal is to progress as quickly as possible.
I have nothing to add except I am jealous of the local conditions and time available to foil 7-10 sessions a week.
What foil is he on now. If it’s good enough I will say yes that’s the one he should be on. You’ll be happy. If it’s not so good I’ll say a code s or any other recent foil.
His weight and height would help too. Kids honestly learn on anything though.
Foil size may also be a factor, he may not weigh enough for his current setup.
If your son is in the most basic early stages - then I wouldn’t overthink this first year! (i.e. Buy him the foil with his favorite color?)
Generally - you want a foil that is higher aspect but forgiving. I’m most familiar with Armstrong … their HAv2 and now UHA (aka HAv3?) are easy to pump, and the HAv1 and HAv2’s are what set a number of the early linking records when that was more of a thing. But there are lots of great options from every major brand at this point. 
I don’t think you can go wrong with the Unifoil Progression for learning to pump and connect waves. At 165 lbs (75kg) the Progression 170 was my go to foil for learning.
I agree that the Unifoil Progression is super super easy to learn on - and to connect waves with. I went from never connecting waves to getting consistent 3 for 1s almost immediately.
That said, I now ride AFS almost exclusively and their wings are so. much. more. fun. to ride. Specifically the silk.