Can someone else see my messages on his device?
You’re mixing up two different thing all happening on the back end of the messaging servers.

You receiving someone else’s messages doesn’t mean you can read their’s.  You don’t know who got you old number and you don’t know who’s old number you got.  The probability that the two of you just happened to swap phone numbers is highly, highly unlikely.


Is it possible that someone saw some of your messages prior to changing the number in iMessage?  Yes.  You don’t know who sent you messages intended for you to your old number.
the messages you got when you changed your number were likely queued up SMS messages that couldn’t be delivered until your number was activated.
is someone possibly seeing SMS messages sent to you at your old number?  Yes.  Apple has no control over SMS messages and has no idea if the sender meant to send it to you or the new owner.


Changing phone numbers is not a normal, everyday process.  Just control the variables you can control and ensure your account information where your phone number is relevant is accurate.  As far as someone having access to see all or even past messages, no, that’s not possible.  Unless they have your iCloud account and can log into it, they can’t access anything.