It seems as though you a bit between a rock and a hard place....
He has said he doesn't share well and he likes his GF's to have no one but him. He realizes this is a double standard but he feels as he feels.
and
I need to be in a relationship for a number of reasons.
First off: He does realize that this "double standard" is hypocritical, right? Yes, he feels as he feels (as do we all), but has he addressed the underlying reasons that he "feels" this way? Is he even trying to work toward overcoming this?
For a long time MrS was okay (feeling-wise) with me having relationships with other women but not other men...intellectually he realized that this was irrational but...there it was. It wasn't until the situation smacked him in the face that he had an epiphany and a major perspective shift - whereas his "feelings" were able to align with his "thoughts" on the subject.
Secondly: Why do you "need" to be in a relationship? (You don't have to answer, this is "food for thought.") Will any relationship do? For instance, could you STOP being in a relationship with your current SO when he moves but leave the option open for resuming the relationship when circumstances align themselves?
For me, I have no "need" to be in a relationship - I feel like I am "complete" whether or not I am in a relationship. Relationship(s) enhance and improve my life but are not necessary for me to be happy.
How can I get him to support me having a BF while we are apart. I would be completely up front about everything with everyone as I will not lie to or about anyone.
It's good that you have a personal boundary that you won't lie to or about anyone. The truth is that you can't "get" him to support you having a BF while you are apart. You can tell him what your decision is - i.e. "While you are not here, I plan on leaving myself open to developing a relationship with someone else. I won't lie to them about you, but I can't promise that I will not get involved with someone else while you are gone. If that is not acceptable to you then we are going to have to break-up...at least for now. If, in the future, we can pick-up where we left off then I would like that."
I did something similar when MrS talked to me about joining the military when we were in college. I said that I loved him and he should do what he needed to - but that I couldn't commit to being in an LDR, or promise that I would still be "available" when he came back. He didn't go...
Not sure if this is helpful, just my take on your post...
JaneQ