I you are putting him in a tub it should hold humidity nicely if you don't put too many vent holes. If you had a tank with a screen top obviously you would need to mist quite often but if you have a plastic enclosure with a few holes to vent but don't overdue them the humidity should stay in for a few days. I got my pair in home made tanks with sliding glass doors on the front and the walls are made of plastic so it is pretty much enclosed in beside the little gaps from the glass and I added a couple rows of vent holes on the back of the tank. There is enough for air flow but it is still pretty contained. I only use paper towel substrate and I only need to mist really well every few days. The tank doesn't always look "damp" but you can still feel the humidity if you open the glass.
Also you should really put a humid hide in the enclose. I got one in there so they can always get that extra humidity if they want even if the tank is freshly mister or a little dry.