I don't have a son but apparently that happens to allow it to grow and to stretch the foreskin (if not circumcised) so maybe I would say that its to help it grow. At 2.5 he probably wont be asking for much detail just a simple answer.
First of all don't panic, or lie. Just be honest and explain it's natural and happens to all boys and not to worry about it. It's what we've told DS and he is happy with that.
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