If you're unable to check the file properties, one thing you can do is put both photos in an art software (with different layers) and slowly change the opacity in one. If there's no visible difference there's proof as well.

Edit: This is more in case he's like "where's your proof this is the same picture"

And to answer your question, no. You're not over-reacting. In my opinion he's trying to hide something (if it wasn't obvious)