6 month puppy struggling to poop outside

I have a 6 month old puppy that was adopted at 2 months old. We live in an apartment building in NYC, so we immediately began training him on puppy pads as we don’t have a yard for him to go in. He picked that up super quick. After about a week of pretty consistent praise and direction, he was good. Would go to the pads to pee and poop without even alerting us.

After he got all his vaccinations, we started walking him, but he wouldn’t pee or poop outside. Obviously it wasn’t ideal, but we had the pads, so we weren’t too concerned about it (mistake, I know.). He would walk for 30 minutes or so after a meal, have no bathroom movements, then after coming back to our apartment, he’ll go straight to the pad and pee and poop.

Starting last week, we’re now trying to make a more concerted effort to get him to go to the bathroom outside, and he’s gotten to be okay peeing outside. Pooping, however, is a different story. He’ll walk around outside like he needs to poop (he’s sniffing like crazy, his tail is all curled up, and he’s doing the waddle), but he simply won’t go. After 45 minutes of this, I take him back inside and he immediately goes to the pad and poops. The few times he has pooped outside, we’ve done big praises and pets, and we’ve tried giving him his treats, but he just ignores them, which also makes reinforcing going outside difficult.

My partner wants to leave the pads down because I go in to work 5 days per week and he works from home, but is unable to let him out consistently throughout the day due to the nature of his work. I didn’t like this idea, but we compromised by placing a pen around the pads and only opening the door and giving access when I’m not at home and he’s at work. Otherwise (mornings, evenings, weekends), the door is closed and we’re diligent about taking him out when he paws at the pen to get to the pad.

Since we don’t have a yard, most advice I’ve seen (saving the poop and putting it in the same place, walking in circles in the same spot, setting up a pen) probably wouldn’t work, but I’m open to being proven wrong about that without my neighbors hating me!

What should we do?

Any advice would be welcome!