When my older nephew was young, he as much like the Dr's son. He NEVER stopped talking. He was a very sweet and loving little boy, but OMG, he NEVER shut up. And, 99% of the stuff he talked about was dinosaurs because he was obsessed with them. He was never mean, rude, always listened, and was actually incredibly smart for his age, but oh my.
Having never raised any kids, it makes me wonder if part of it is not just the difference in boys and girls. Boys tend to be much more confident at a much younger age. It seems like he does have a tendency to test boundaries and I agree with G, that the Dr seems to want a father figure to help with discipline, which is kind of a double-edged sword. If you all do continue forward, I'm sure you will have discussions about boundaries and how to handle each other's children, but it may be harder to navigate on the front end when you really aren't in a position to have any say.
I think what stuck out to me most or what would put me on edge most if I were in your situation is something you said in passing about how the dr doesn't really correct him because she doesn't want to "stifle his creativity". (I'm paraphrasing here, so forgive me if I didn't say it exactly like you did.) Having worked with children of all ages for most of my adult life, I have found, in my experience, that it is usually the parents who say stuff like that who have the kids who are hardest to deal with. I am ALL FOR kids being creative and having an imagination and playing and all of that because it is part of how they learn and grow and interact, but many parents use that as an excuse to not really parent and that is not good for anyone. Children need rules, boundaries, guidance, structure and not giving those things because it stifles creativity can lead to a kid developing a pretty good sense of being a jerk. Not saying the dr's son is, mind you, because you have described as a nice, happy kid, but he will be a teenager at some point and while his some of his behavior might be cute now, it won't be so cute when he's 15. One can correct a child without stifling them.
Something else you said, though, makes me wonder if how the kid is maybe more of a product of not being overly socialized. You talked about the structure the dr has for him and that he doesn't have a lot of socializing time with anyone but the dr, outside of school. Little kids need rule and discipline, but they also need interaction with others so that they learn how to work and play well with others. Maybe you and your girls can be helpful in that because you give him other people to interact with and he can learn how to better handle himself and dealing with others.
Me 52, H53 Bomb drop 9/29/2014 Divorce from XH final 12/17/2014 Marriage #2 12/31/2019 5 adult (step)daughters (3 from XH's first marriage, 2 from current H's previous relationships) 6 grandkids