Can't get over this little headcanon about little Will was actually non-verbal for over a year after they met, but Mike doesn't remember it, because in his head Will always was communicating well with him.
He still thinks Will said yes, when he just nodded. Mike always talked for two of them. And he still asks are you okay, because that's how he managed to understand Will in the first place. All Will needed to do is nod in response.
When Will started speaking, Mike almost didn't notice, but Joyce overheard it from the hall and couldn't stop crying until Jonathan came back from school, and then both of them were standing behind Will's bedroom door in awe, listening Will talking for the first time in years.
This is it. This is my headcanon.