I've recently had this gorgeous little dog come live with us. When on walks he goes crazy when he sees other dogs. He barks his head off and is impossible to move on or away. Treats just aren't distracting him. Though we've not tried training with treats in garden 1st before moving it to the great outdoors! I'm considering a halti headcollar - not optifit as that seems to gave issues with an adjusting clip.
We had a similar problem and had been using one. After a spell in kennels, one of the kennel workers (very experience in 'problem' dogs) told us that made it worse because the tightness stressed him out even more. Sure enough, we've found that keeping his lead as loose as possible but physically close to us when he spots another dog has made a huge difference. He still reacts badly when he sees a small dog with it's tail up (even if it is just built that way), but it is way better.
We were also advised by the vet to start getting his attention with treats in the house for a few days before using them outside. However the kennel bloke said that just reinforced that there was a 'thing' going on, so we've pretty much stopped that.