Good advice from Sara and DevonW above. It's a tool, but yet one more in your arsenol - and it could help. An example: we recently were in a Nosework trial, and parked in the "reactive dog" section. The dog next to us was another GS (funny, they were calling it Germ Shep Row .. the 'reactive dog section' turned out to be a row of reactive GSs
) - anyway, she had her GS in a Thundershirt. We got to talking. Her girl was reactive to other dogs, and she's done loads of work, but the T-shirt has proven to be a huge help. Naturally, at a trial, there are loads of people walking by with dogs (altho no interaction) - so her girl did her searches in her Thundershirt. The dog next to her wore one also - it was reactive to people - again, she had done loads of work, but in a situation such as a trial where there were going to be loads of people (but no interaction unless asked for), it's proven to be quite helpful.
Will it solve your problem? Most likely not. But adding one more tool to one's arsenal is always good. Try one, continue to do loads of socialization, counter-conditioning, desensitization, etc - and it may help Dan to feel more secure while he tries to figure out this crazy world. As far as finding behavior to reinforce, keep in mind that sometimes we look for or expect too much. Reinforce even moments. A moment of good behavior - click/treat!! Sometimes one moment (translation: a second, even a nanosecond) can morph into 5 or 10 seconds, then a minute, then 2 minutes ... and longer.
Hang tough!! It'll be ok.