Your time pressures are not helping things go forward and I think the problem itself has two causes. I think it is a combination of misunderstanding on the dogs part and a desire to feel more secure.
Try getting up ten minutes early for a week and putting a short leash on the dog before you take him out to pee. Leave the leash on him when you come back in and watch him like a hawk, without talking to him or looking directly at him. As soon as he shows any sign of needing to poo (it may be useful to wait until he has actually squatted) grab the lead, say a very firm, "No!" (Don't shout or sound angry, the more of a drama you make of this the worse it is for the dog to try and sort it out) and walk with large confident strides outside to the area you would like the dog to use. Then, in a softer voice, but not looking at the dog, give him your usual instruction, I use, "Get busy, busy busy." Give it five minutes and if nothing has happened just go inside and try again. Try and keep your heart rate under control, it is vital that you remain calm and in control no matter what happens - an awful lot easier to say than do, I know, especially with an ongoing problem.
When he eventually goes outside have a food reward, a toy and loads of physical praise ready.
Keep at it and try to ignore mistakes and carry on as though they haven't happened - not easy!
Good luck.