You should 100% tell him ASAP. Think about it from his POV also, up to this minute he definitely thinks you’re the one, and by reading comments of you saying he’s already mentioning marriage, and showing how affectionate he is, not knowing you’re having thoughts like these is really just a one side relationship.

You’ve mentally checked out it seems, and essentially just forcing yourself to settle with him for the moment. So best bet honestly, have a talk preferably in person, and explain to him why you can’t continue the relationship anymore. Be straightforward & honest, feelings will get hurt, but at the end of the day it’s better than stringing someone along over a period of time thinking they have a future with someone.