Thanks for your points, there's some really valuable stuff here.
I'd say he's in his early 40s, and I try not to be ageist since I've had some outstanding older developers that I've worked with, but I think he's perhaps stuck in his ways a little bit.
Unfortunately, it's just me and him which are building this new API with no other developer involvement. So it's kind of like a "he said, he said" scenario. Another unfortunate to pile on top of that is that we don't work in Agile sprints (I've worked that for the past 4 years so it's quite a change for me), so I only speak to the other developers once every 1 - 2 weeks. The only daily contact I have is the "Senior" who is in charge of this specific Laravel project.
I love the idea about sitting with him and talking. At first, he seemed quite cold to me. It warmed up a bit last week but now it's back to cold, so I'm not sure if I just caught him on a good or bad day, or if I'd said something to upset him.
He was pretty firm about not doing either the OpenAPI or Composer things today, I tried to push a little bit, politely, and just say in the nicest way I could that Google Docs wasn't the best fit for what we needed and that we'd probably be doubling-up on our work in future. He seems very focused on the time the project will take, and it feels like he sees any suggestions as a burden.