Tell him. Don’t tell me - tell him.
If you’re losing feelings for someone, it isn’t fair to either of you to keep pretending that everything’s alright.
Also, do not act on your feelings towards swim team guy until you’ve had a proper, grown up conversation with your boyfriend and you’ve gone your separate ways.
For his and your integrity’s sake, you may want to give it a few weeks before you start dating again.
Good luck (: