problems with potty training

I have a 1 1\2 yr old pitt. I had a hard time potty training him. Its been about
2 months since hes gone in his crate. hes doing pretty good. The problem is when I put a bed, or a towel down for him to lay on he pees on it. I have tried new towels, new beds. Within an hour he will pee on it. I come home from work every 4 hours to let him out and hes fine, unless there is some kind of padding in the crate. Recently I have been leaving him out while I leave the house for short periods of time, like an hour or so. He has had no accidents. He can go for 5 or 6 hours in the bare metal crate. Hes also good all night long, he has his own bed in my bedroom. What might cause him to go only on the bedding in the crate ?????