Hmmm. It can take them a little while if they are holding it to get to the point where they can let it go. How much are you walking him? One of the best laxatives is a good, vigorous walk. Is his diet regular? Too many chicken drum sticks can constipate. Has he picked a poop spot? Habit is your friend. Weather can impact poop comfort. Deep snow can cause an issue (how and where).
Are you encouraging regularity? For example when something goes in the front end, he needs to go out because something should come out.
Are you teaching him a potty queue? We use "do you have to go potty?" and an excited reaction gets "potty outside" then we remind everyone "go poop" then there is cheering for success. We still do this for our ancient ones. It's a ritual. Can your older dog "demonstrate"?
I think if it were me, I'd take him for a loooooong walk this week end or in the evening until he poops then make a big positive deal of it. If you are doing clicker training use that.
Yahoo answers has a few ideas: catch him in the act and run with him outside then praise the completion of the act. Save an indoor poop and bring it with you to help him go what you want, where you want. Wipe his bottom with a warm washcloth (like his mama).