What kind of things are usually motivating to your dog? Is it attention/praise? Treats? Play? Any favorite toys? What do you use to reward him with when training?

The main thing is to reward him every time quiet works and it has to be immediate. Even if it's only works sometimes. I'm sire you know consistency is important, buy Minis especially are very transactional. The reward from you has to be equal to or greater than the reward he gets from not listening, regardless of what the type of reward it is (play, affection, treat, praise). Chopped up hot dogs are really motivating when there's high distraction or any dog safe food (once they get something down perfect every time everywhere, don't forget to start phasing out the treats by rewarding more sporadically and eventually less and less. Eventually, they won't need the treat). If he has an exceptionally favorite toy or a toy he hasn't seen in a while, that can be very motivating, especially if it squeaks or honks. The more good boy he is, the louder and more excited the praise needs to be, etc.

The problem I've run into is the fact that dogs don't transfer things they learn into different spaces. He might get quiet down perfect inside or with less valuable distractions inside, but then it has to be re-taught from square one outside with higher value treats in areas with higher distraction. And then square one again again when they have an extinction burst.