Well put it anyway that works for you. If necessary just voice how much you love to hear from him x times a day, week, whatever. And just ask him what is reasonable to him. If he comes up with reasons for not communicating on the level that you would like, hey you're still ahead, you've gotten a reason to help you when you're bummed. You'll know that it's this reason or that.. (like he can't talk as often because meetings or something.. etc.. whatever the case may be) Communication no matter what is what's going to help. Even if it doesn't change the amount that you talk to him while gone, you'll understand each other better, grow closer and that in and of itself is worth it.

On vulnerability, the best way to be as close as possible as you can to your SO is to be willing to be vulnerable. it's hard but darned well worth it. This is one thing I believe that is a part of why people don't feel as close to each other years down the road. Being unwilling to give all of yourself to your loved one. Just thoughts on that. I know it doesn't make it any easier but maybe it will help to give you reason to do it in spite of your fear.