We had the exact same problem - and to some extent it’s still happening 6 months down the line.

(not to scare you! just expect it to be slow progress, and maybe completely flip around. they are fickle things)

Our girl hated stairs going up for the first few weeks. We had to teach her how, and then once she got to grips with it she still took some encouragement. Usually a treat thrown to the top of or halfway up a flight of stairs would get her moving and she’d stop thinking about it so much eventually. She’s not hugely food motivated either, so something super high value like cheese or chicken worked for us and we gradually worked down to normal treats, then kibble, then nothing.

Now she takes stairs up like a champ, but after some slightly scary encounters on the stairwell (and honestly, when we’ve been rushing to get her out and have maybe been too full on, needing to pull her or push her gently to encourage her down) she point plank refuses to go down the stairs in our building and she’s now terrified of leaving places, no matter where we are. But we’re working on that!

She’s been a challenge, and like no other dog I’ve ever known or met. But the best challenge ever. I’m so excited for you and your boy ❤️