If you intend to continue this relationship, you need to bite the bullet and tell him the truth. Relationships based upon lies are living on borrowed time — sooner or later the truth comes out. Based upon the fact that you told him you were 18 a year and a half ago (when you were 17), I assume you are now 18… going on 19?
I have no idea how old this guy is, but assuming he’s within a few years of your age I suggest that perhaps you make a joke about turning 19 this year, and fess up. If he loves you, I doubt one year is going to make that much of a difference to him — unless there’s a big age gap.
Frankly, lots of women lie about their age, albeit it’s typically when they are a lot older… claiming to be younger.