How can I convince my Sheltie to potty on dirt?

I have a 1 yr old Sheltie, male, not neutered, who refuses to poop or pee outside unless he's on grass. I just moved to a new place with a fenced back yard that doesn't have grass but did have some ground cover. He would go out there for the first 3 weeks until I pulled up the groundcover because we're changing the landscaping. Now he will only potty outside if I let him in the front yard that has grass, otherwise he will hold it as long as he can and then go in the house. Until about a week ago I couldn't even remember the last time he had an accident in the house. My other dog, a 7 month old boxer mix, will go in the back yard no problem. I have tried letting the Sheltie out by himself, with the other dog, with just me, with me and the other dog, on leash, off leash, EVERYTHING, and he will not potty even though I know he needs to go really badly. If he does get on grass he pees immediately and for a long time. How can I convince him that it's OK to go on dirt/mulch?