Be honest and let these other guys know that you are no longer interested in dating them.
Now that you have finally decided to be exclusive with the one guy, you should end your relationships with the other guys that you have been dating definitively.
He doesn’t want to put a label on the relationship and may be worried that if he were to make it official, there would be a new set of expectations that you may have of him.
He may not want to feel that kind of pressure and is happy with the status quo.
This guy has been dating you for 3 months and hasn’t had to make a true commitment.