I would start by walking them well away from other dogs.
Get Rupert to where he can tolerate just seeing other dogs in the distance. I would make the appearance of another dog on the horizon the cue that he's going to get some really good treats. Be quick about it: be sure he gets the treats BEFORE he starts going spastic so that you don't inadvertently reward anxious behavior. When he's comfortable with that, you can - very gradually - try to reduce the distance. If he freaks, you're going too fast. Back up a few steps.
I'm not sure that Rupert is ready for a play date.
Good luck.
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