A New Fearful Behavior from our Tuff Guy

I've been meaning to post this for some time and just haven't had the chance. I'm looking for suggestions and ideas around a new behavior Zuko has been showing.

Obligatory cute photo of Zuko

I don't know what triggered what, so here's the full chain of events:

• Zuko has always been fine with other dogs in his house. Usually we have the first meeting be outside and even if there is an initial growl, things are fine after. He doesn't mind other dogs playing with his toys, sniffing his food bowl, drinking his water - he's a very polite host in general. 
• On a Friday in October he had his teeth cleaned. This involves being "put under" but the vet reported no issues. He was fine that night but only eating soft food.
• The next day he seemed a bit lazy but fine, so our friend came over with his dog "L". Zuko and L had never met and she is a rescue dog with issues of guarding her human. We could not do the intro in the backyard because it was dark and pouring down rain. There was some serious growling and snarling when L came in the door (she was still on leash) and then Zuko spent the rest of the visit trembling behind my legs and L's human's legs. Final note: L peed in the kitchen before she left. 
• A few days later we met a boston terrier at a restaurant we have visited with Zuko many, many times. It has an enclosed patio and we sat in the same spot we always sit. Zuko has meet "P" before and had no issues. This time he freaked out. Snarling at first encounter then he was hiding and trembling behind my legs and trying to escape out the nearby door. Finally I had to put him in the car where he went to sleep.
• About a week later we went to Mendocino with friends and stayed in a rental home. Their dog "T" is also familiar to Zuko - they've met several times in both her house and ours. This rental home was new to both dogs. Zuko started the same behavior - when T would come up to him he would dip his head, look away and move to another area. Part of the issue was T would not leave him be - she was in his face always. When we would put him in the bedroom and shut the door, he would calm down and be fine. This is also the weekend he ran away - details are here. 
• Another week went by and our friend came over with her boston terrier puppy "W" who Zuko also knows -- and he knows this friend very well. She has watched Zuko for us in the past when we traveled. Same behavior - Zuko was trembling and was scratching at his hudad to do something (we don't know what). Except this time W was not even near Zuko. He was off exploring the toy box and left Zuko 100% alone. 

So what do we know for sure? Enclosed spaces are an issue except his daycare - they report no problems there so it may be enclosed spaces with either his humom or hudad. Dog parks seem fine and he was fine on the beach with other dogs until he lost his off-leash privileges (we have not been to the beach since). One other note: all of the other dogs involved in the above incidents are very high energy. 

We have puzzled over this - what was the trigger to cause it to start (if any) and what is the "in the moment" trigger? Is it us? Is it the other dog? How do we get past this? We have a trainer lined up who works at Zuko's daycare but I wanted to reach out to the Shiba community and see if anyone has had a similar experience. Our fear is we try this, try that, try something else and just confuse Zuko even more, so while we know about Thundershirts and the Comfort herbal things, we have not tried them. Any insight is appreciated!

-- Posted by Prince Zuko's Puzzled Humom