Boullier says he has struck a strong bond with Hasegawa, who was part of Honda's last F1 project with BAR then as a factory team, following his appointment as the head of its current programme on March 1.
GARY ANDERSON's half-term F1 report card
Now in the second year of their collaboration, McLaren and Honda have made progress on- and off-track this season, following 2015's tough reunion.
"It is a completely different approach with Hasegawa-san," Boullier told Autosport.
"I've a very good relationship with him. We meet very regularly, face to face.
"That is not only at the track where everybody is busy and has their own duties, but we meet very regularly anyway, and he is open, transparent, pushy, clever.
"We are starting to be co-operative and collaborative as well, so you can see some projects where McLaren is part of it, but we hide behind because everybody is doing his own duties.
"But it's working, and that's the key."
Boullier feels McLaren and Honda are "pushing as one team", with the Frenchman genuinely believing they are now "not far" from making a real breakthrough.
"McLaren and Honda balance," added Boullier.
"We had to push first to make our car good enough at a certain level, and we still have a lot of room in making the car better, and we know that, which is good.
"With aero, suspension or whatever, we know what we have to do, and we know where to go.
"Honda is now starting to pick up, and that is why we had a good upgrade at Silverstone.
"We have a lot of development possible, which we can't use now, because, for example, it brings downforce, but it brings drag - and today we don't need drag on our car if we want to stay competitive.
"It's like we have a couple of developments on the shelves, but we can't bring them now because we want Honda to step up one step, so we go one step, Honda one step, and so on.
"This is why it's a collaborative approach, but it is coming."
It is anticipated Honda's next power unit upgrade will arrive for the Belgian Grand Prix after the summer break, or potentially the following weekend's race at Monza.
While the token system that has shaped development of the turbo-hybrid systems since 2014 will be scrapped at the end of this year, Honda will still have to work within a limit of four power units per driver per season.
"[The token system] is one less restriction in the system," said Boullier.
"It gives Honda the freedom to do what they want, when they want. Still, with less engines used per season, we cannot use an upgrade when we want.
"We still have to be mindful of what we do."