Do you use a cue to tell him when to go to the bathroom? If you don't, start training one. Take him out front for now and give him the command(whatever you'd like it to be, I use "go potty" lol), and as soon as he goes, act like he just did an amazing thing, maybe the best thing ever! Once he's got that down, take him into the backyard, on his leash. Don't let him go around the whole yard, stand still in one place and give him the command, and wait for him to go. It might take a bit, at first he'll probably try to pull you around the yard. But don't give in, you'll have to teach him that potty time is all buisness, and he can't do anything fun until he's gone. Once he has, make an even BIGGER deal out of it than when he went in the front yard, you could even give him a treat or two, or play a fun game of fetch of he loves that 
Good luck
~IJMB
Good luck
~IJMB