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Hello. Thanks for the question.
This is not uncommon at all, especially in northern breeds. They will often see small dogs as a prey species and that trips the predator/ prey drive in them. We see this all the time here at my ER where small dogs are picked up and shook by larger dogs.
You can train that out of him. It'll take time though to be around smaller dogs while still maintaining a normal state of mind and paying attention to your cues. Make sure this is all done on leash until he is fully trustworthy before doing any off leash work.