Could just go for straight up honest conversation. Something like:
"I am okay with you exploring your needs, I really am.
I just think asking me to be you "main hub" person over Long Distance Relationship for 12 mos is not for me. I do not like LDR. I just cannot see me being okay with someone else being able to see you and spend time with you while I am stuck on a computer/phone. I rather be Closed until we are local if I LDR'd at all, and you rather date other people now. We are not compatible.
I prefer to stick with my decision to walk away before it just gets even harder to. Then I spare myself any future angst over LDR problems and you are free to date who you wish. You could look me up when local and we see what we see then."
There. Problems solved. If he DOES move in 12 mos, and you still want to date him then at that
time, great. Date him then.
But at THIS point in time? You wanted to end if for a reason. Could let it BE ended for a while and see what that feels like and see if it serves you better.
You have every right to have your relationships in the manner you enjoy best. So's he. If they line up, great. If not, it's not. That's what the dating time if FOR -- to find those best compatible.