Hi Joso,
I just saw your post. Is there anyone you know with a placid dog who you might be able to let him off with and test the water?
If you did it somewhere contained like in a garden or a fenced area, then it would be fairly easy to get control of the situation if it didn't work out well.
It sounds like he might be fine, but I can understand your nervousness to try. What did your behaviorist say?
This is only my suggestion, and I am by no means an expert! But if it were me, I think that's what I would try, as long as I knew it would be a quick job to get him back on the lead if it didn't work out. And obviously continue to apply the 'treat' rule if it's something you've been using generally...
I definitely wouldn't involve toys as that would probably be a disaster!
Also, consider that he might just be fine with some types of dog and not with others. (Although I guess with a jack russell the majority are going to look larger lol) My dog loved dogs his size and smaller, and was okay with large dogs, but when they were large and bouncy he went on the lead immediately as he didn't seem to be able to read whether they were being playful or aggressive.
And you could always contact a trainer, who might be able to help you with testing out group situations off the lead?
Good luck anyway, sure it will work out well :)