Why won't my boyfriend let me call him by his childhood nickname?

My boyfriend and I have been together about 6 months, and we took a trip recently to visit his parents (they live in another state). I noticed his mother and sister call him by a nickname, and I asked them about it and they said it's his childhood nickname and that they always call him by it.

I thought it was really cute, but when I called him by it when we got back home, he asked me to please never call him by that, only his mother and sister are allowed to.

Am I right to be hurt by this?

Why can't I call him by his nickname? :(
Why won't my boyfriend let me call him by his childhood nickname?
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