I agree with everyone here, I don't think you should be blaming the aspen prematurely. I'd venture out on a limb and say 93% of corn snake keepers use aspen exclusively, and most of them never have shedding problems aside from a snake in poor health to begin with.
I think Caroline was accurate in saying if he was kept in a humid enclosure with the coconut bedding for 3 years, he's probably adapted to it. And when put on aspen, needs time to adapt to a drier ambient humidity level. My relatives live in Western Montana where there is no humidity. They come here to Southern Indiana to visit and die for a couple of days till their body learns to adapt to it. Same is true for corns I'd imagine.
If your snake had one bad shed after being on the aspen for a short time, I'd give it a few times to make sure that was the problem before advocating it not be used. If you had a humid hide available with damp moss, then I don't see why he should have shed badly.
My house is commonly 30% RH due to the AC being on, but I very rarely have bad sheds from my snakes, and I don't even have water bowls big enough for them to soak. And the only one recently that I can remember was an 04 amel that had gone off feed for no particular reason for a few weeks and had shrunk down to nothing and then tried to shed. After she started eating again and putting weight back on, her sheds have improved to the point that they're a one-piece shed again.
As for potting soil, I don't think I'd recommend that at all. It would risk impaction from it being on/in everything due to its small size, it would clog up the nostrils from a burrowing corn, could cause infection, harbor bacteria from fecal matter, half you can't find anymore without fertilzer or perlite additives anyway. And putting a misting system with corns would be totally unnecessary. Corn snakes are almost always found on the ground and a snake that's kept wet will develop scale rot and other issues. Misting units belong in a lizard enclosure where the lizard is on the branches/vines and not in constant contact with the wet substrate. Even tropical snakes with misting systems I've heard of issues arising from belly rot.
What is wrong with aspen?! Its easy to obtain, easy to clean, virtually dust free, looks nice, non-toxic, I could go on and on. Unless you liked newspaper or paper towel, I don't see why one wouldn't use aspen for the more 'naturalistic' alternative. Repitbark doesn't work so hot with corns unless you rinse it thoroughly beforehand. Its red dust clogs up a burrowing corn's nose and mouth. It looks nice, but still I can't justify the beauty of my corns for their long-term health.