Ferrets can be VERY difficult to litter train. Really, what you're teaching them is to prefer a certain substrate, or interrupting where they go. As far as I've found, you can't teach a ferret to find a specific area or a specific item (A box) and go there.
What you can do, is put a litter box everywhere he DOES go, with a specific kind of litter that isn't duplicated in your home. If you have carpets, no Carefresh. A bin with wood pellets for the stove, no pelleted litters. Clay litters can be very bad for ferrets, they like to dig, and it can clog their nostrils and dust can cause respiratory infections. Find a litter that's right for your ferret and home.
And that's about it. Ferret litter boxes are put where ferrets go to catch ferret poo; ferrets don't seek boxes to put their poo in. I have a 2-level Ferret Nation cage with three female ferrets and three litter boxes in the back access corners. I still catch them pooping willy-nilly all over the place.
The easiest thing to do would be to get him a cage- if it's going to be temporary and he's going to get lots of exercise, it can be smaller- and put a litter box in each corner. Make them low-sided, extra large corner pans. Hang hammocks for him. After a few weeks remove the one box he uses the least; then the next; then the next, until he's down to pooping in just one box in just one corner at least 90% of the time. If he keeps pooping in the corners the boxes were removed from, or he misses the edge of his boxes more often than he hits, put more boxes back in.
The day he's almost-perfect, let him back out, with litter boxes in every corner he used to poop in. Start him with access to only the bathroom; then the kitchen; then the living room, and etc. Until you have him doing what he did in the cage, reliably using only a few corners with boxes in them for pooping.