Do you walk him regularly, or does he just go outside periodically? Reason I am asking is, when I walk my 1.5 year old or when he is outside for 20-30 minutes at a time, he goes poop more than once--sometimes he goes 3 times. You may need to do a walk or outside activity for awhile after meals or if you have been gone for a long time, and particularly in the evening so he can get everything "out of his system."
As for the steps, his scent remains there even after you clean, and so that has become his designated "spot." If you haven't, try one of the enzymatic pet cleaning products to try to neutralize it better. But make sure you work out the pooping schedule first or he may just start going elsewhere. At least now you know where to look for it.