Sounds like a routine vet check is in order.
But that being said, I have to wait for my female boxer to go and poop outside. She does NOT like pooping in the cold, especially if there is snow on the ground.
If you walk him for say 10 or 15 minutes and he is just refusing to poop, bring him in and keep him on the leash. Keep the leash attached to you (around your wrist, on your belt, wherever) and then try again in about 10 minutes. Dont let him get away from you and get the chance to poop in the bathroom.
With my boxer Heidi, we adopted her about a year ago. She just cant be trusted to free range. If we let her, she'll poop in the house (even if she JUST pooped) Its her weird little quirk that we've never been able to figure out.
If we walk outside and she refuses to poop (when I KNOW she has to, her little butt pops out when she's gota go) I will either keep her on the leash or put her in the kennel and try again in about 30 mins. She doesnt like staying in her kennel when everyone else is out so she usually goes RIGHT away as soon as we let her out.
Once we got really strict with her pooping outside or she goes in the kennel for a bit, she started putting two and two together very quickly!