It signals he feels the "yes" very strongly.
When he does it everytime it suggests that he feels passionately about everything he says yes too.
It might mean he's enthusiastic about you.

Or it might mean he holds his finger on the "s" key a bit too long.

He either finds you annoying or is bored with you

It's the snake in him coming out....

Yes but extra enthusiastic.

Does he look like this?

A sticky finger?

slurring ... must be drunk... 😏

he is a snake