By the way, people on Twitter think Go is an entitled bastard for rejecting (at first) a Scorbunny who practically searched high and low for him in favor of getting Mew first.
I feel they are over-reacting a bit too much.
Go clearly looked unconfortable with turning Scorbunny down at first. It's clear from the way he worded and how he talked to it.
Also, it's important to note that while we, the audience, know what Scorbunny had passed through, Go had no idea. For all he know, he just assumed Scorbunny followed him up until that point.
All things considered, the scene was handled fairly fine.
As for Go changing his mind... I'd say it was kind of gradual. He seemed to be moved with Scorbunny following him all the way to the Wild Area - thus why he looked unconfortable with turning him down unlike what he did with the Kanto starters on episode 2 - and then he saw all the effort he put to help him with Snorlax. All those caused him to finally decide he should rethink his decision.
By the way, the Ivysaur episode did have Go at one point looking dissapointed he couldn't help Ash with TR since he had no pokémon. Maybe that added to the above points as well.
That's how I see it, anyway