30? Then it should be an engagement ring. It's the adult thing to do. (What exactly is he promising? To one day replace it with an engagement ring? Then a wedding band? If he wants to give you a little token of his loyalty & friendship, a favorite sweatshirt, baseball cap, picture, book, letter or momento might be more appropriate, jmo.) (If you decide to keep it, maybe wear it on a chain around your neck, vs. finger? So serious men, with solid interest, know you're available.)
A promise ring, when you are an adult, represents that your partner will be faithful to you, honest with you, caring, etc. now and in the future. Usually intentions are stated when a "promise ring" is given.
Promise rings are often worn to represent an individual's firm pledge to be real, either to someone, a code of recognition, in order to a spiritual belief. The most typical reason couples exchange promise rings would be to pledge dedication to being engaged later on.