Been with my boyfriend for 3 1/2 years?

Were pushing 4 years. We got together when I was 17 abd he was 18 now i'm 20 and he's 21...I'm going on 21 and he's going on 22.... I love him so so so so much... he is my first real boyfriend, but he has had plenty of girlfriends befor me...My question is why if I ever bring up marriage in the farrrrr farrrrr future he shy's away and dosnt really answer but he tell's me he wants to be with me forever and have kids some day... he proposed to me when I was 18 and I said no because we are way too young... but I dont want to waste my time if he dosnt want to marry me,,,, what;s his deal?!