Struggling to get puppy to go outside

Hi everyone,

We have a 5 month old puppy that we are struggling to get to like going outside. We carried him outside for the first few weeks we had him because he was still learning how to walk down stairs. He would do his business outside just fine then dart back to the door to go in. He’s now comfortable with stairs (the inside stairs are now his personal agility course, ha ha), so I know it’s not that. We’ve tried treats and celebration when he is in the yard. We’ve also tried high reward treats just for outside, but there is no treat we’ve found that the reward is high enough. Won’t even go near the door on his own. Just wanted to see if anyone had any other advice.

Not sure if this is related, but could be. He was older than I’m used to when getting a puppy from a breeder (18 weeks) and I don’t think he actually went outside much while living there. They had a really nice deck built out to an artificial outside area.

Thanks in advance!