Perfectly Behaved At Home, Ignores Us Outside?

[chichihuahua]

Hi all, I have a bit of a problem with my dog.

He's perfectly behaved at home - listens, sits on command, waits for food etc etc, however the minute he is outside and he spies another dog or some really obese person (I think they scare him? He seems to really get worked up if a large person walks towards us), he ignores me like I don't exist. I try to snap him out of it but he gets all glassy eyed and fixated on the dog/person/stroller etc.

At home he's fine, even if he's one-on-one with dogs at dog parks he's fine too. It's really annoying because I don't know if I'm training him properly or not. We go over everything in the backyard or at home, or even with a friend's dog he's perfect, attentive - but that all flies out the window once he's outside with outside stimulation.

How do I best tackle this situation? My dog is just so selective it's hard to know when he's trained or when he's not! For example, some days he'll take to random dogs he meets but then on other days he will bark at them. It's the same with the people we pass on our walks - some days he'll be fine and won't notice them but some days he'll fixate and bark.

It drives me insane!