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We have some neighbors on our street that also have daschund, but he has always been a very shy dog (his owners treat him like a child)
My sister has two very friendly labs that Rudy has always gotten on with in the past, and has offered to have Rudy for a few days to see if it helps, she is an hour drive away though so regular visits are tough.
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try and meet your sister and her dogs on neutral territory, if he is becoming fearful/shy to other large dogs you need to be gently reintroducing him. forcing the interaction can exacerbate his reaction
does he have a favourite treat/ reward?
if so, start at a long distance. feed him treats, let him notice the other dog and then keep feeding him treats as long as he is calm. if he shows signs of anxiety (yawning, barking etc) move away. it can be useful if you chose a location where there are things that can block his line of sight to the large dog. you may find yourself turning into a pez machine, continuously dishing out treats!
there are various videos and training systems online - look for shy/timid dog training
dancing james
Do you know anyone locally with a confident and stable dog that you trust?