Boyfriend going away for years :( i am heartbroken..please help?....??????

my boyfriend and i have been together for over a year....now he is going away to singapore for graduation ... he has been wanting this since a very long time..and he was really happy when he got through..and i didn't have the heart to stop him even once...we are both very sad about this ... but i am happy that he is doing very well in his life too......we are very serious about our relationship...but he feels that a long distance for a few years will cause us to drift apart and we wont even be friends anymore....deep in my heart i know this is true....he said i can date other guys ( just go on casual dates...not get ually involved or anything )...and we can get together after he comes back...which will take at least four years....he feels its his fault that we are having to break up and he doesn't want to hold me back....i am not scared of long distance but i know that once he gets back....we might not be on the same boat....i am really confused...should i be selfish and stop him from going or should i let him follow his dreams and let this relationship crumble????? I am 20 and he is 19...so we are not immature teens infatuated by each other.....should i carry on with a long distance....we will barely meet btw :( i am upset since a really long time and i think he is beginning to realize that....he might just screw the whole idea of going....what should i do?????