Out of curiosity, Zoom, what size did your son test out and what's his inseam. DH and I have talked, although not very seriously, about getting fat bikes, but my short 29.5 inseam makes it tough. I don't really need one, and am not sure I want one, but I'm curious nonetheless.
As for your son, my advice is to get him a decent hardtail that fits. You'll pay less for it than a fat bike and it won't be so painful when he outgrows it in a year or two. The Mukluk, as you know, isn't cheap. It would be a shame if he quickly outgrows it. And an HT that fits him and isn't a dog like the Cannondale should be just as fun.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher