The line "Let's get ready to rumble" was trademarked and popularized by sports and entertainment announcer Mike Buffer. If you're asking about the opening theme, it's by 2 Unlimited in their song "Get Ready For This."
Ready to Rumble is a comedy film from the year 2000 that was directed by Brian Robbins and written by Steven Brill. The movie is based on the former entity WCW or World Championship Wrestling and draws its title from a famous Michael Buffer catchphrase, "Let's get ready to rumble!"
No one in particular. While Darry and Paul (Soc) were having a stare down, almost ready to start the rumble, Dally came running in and screamed, "Wait! It ain't a rumble without me--" That distracted Darry and Paul was able to take the first swing of the rumble, which was on Darry.
If he says hes not ready then I guess it means that he is either not ready for the relationship or not ready for you. Not to be mean. This is just my opinion. If hes not ready for the relationship then that means that he may not be ready for the responsibility. Hope this helped!