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I can see how Teddy might be appealing for a little boy as it is a very cute nickname. However you don't want to get into a situation where you have become very attached to Teddy but Theodore is now 5/6 years old and ready to grow out of it.
My advice would be to see how it goes, IRL nicknames usually come on their own with no warning and the child themselves with decide whether to let them stick or not. :)

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