What was your life like prior before befriending him?? Do you have any other friends, any other interests and hobbies??
Do you have any other things that make you feel excited, happy, and joyful about life??
If he is your only resource of happiness,
If he is your only outlet to be sociable and have a semi social life..
If you're dropping hints,
If you tell him now, it's manipulation tactics and strategies in an effort to perhaps not revert back to life previously without his company and presence..
If it's not, he may view it as that...
Especially since you have liked him for awhile, and haven't said anything until now.
It seems like you are in "panicked" mode perhaps thinking about how you're gone to occupy your time without him.
Do you feel like you'll be forgotten and tossed to the side? That he'll meet new people??
If you know that he really wants to take this opportunity, don't keep trying to discourage him, encourage him. Stop telling him things like you'll miss him..etc..
It will only be 3 hours away. A short drive, train, plane, boat, ferry, etc... ride to see one another.
If he stays, perhaps if you haven't any other social network and avenues, you may want to establish some independently.. It's good to find more people and things to occupy your time...and both have personal space away from one another.
My previous responses still stand.