I adopted a 8 mo. Old fixed male cat, he goes in the box every time but he doesn't cover his potty.

Be glad he doesn't; easier to clean.
Covering it doesn't make it smell less.

I have 4 cats currently, none of them are any good at covering their mess, even though they do try. My boys are kind of half hearted attempts..
It maybe an issue with the litter you are using and they don't like the feeling of it.
Either that or they are leaving it on display to show how clever they are to use the box you supplied rather than the corner of the room..

I don't think there's anything you can do to train that. One of my girls paws the wall behind the box, or the floor next to it, after she's done. One of my boys scratches the edges of the box.

Like Ben said, just be glad it's not the floor. If you're bothered by the smell, try sprinkling baking soda over the litter between scoopings.