How can i prove i want to marry him!?

my boyfriend loves me enough to get a vasectomy reversal and marry me. he is almost 28 and im almost 22.. i think he feels like because im yuong i may want to go off and be free one day. he thinks i will want to break up due to me not getting to party and do all the things a 21 year old does. ive triedto tell him that im ready for marriage and children one day! he belives me but is still scared considering a reversal is a big decision. im fully dedicted to this man and ive already partied like crazy so much that i do honestly want to settle down with him! if you knew me you would be able to see it more! how do i show him that i love him and want to marry him and never leave?!