How to deal with long distance relationship when he won't be back permanently until December?

The fact that you know there is a date and a light at the end of the tunnel is the best way to be able to make it work. If this person is great for you and you all get along, for the most part, I do not see why this would be an issue. I get that some loneliness might set in but that is where your friends come in to make sure you have people to talk to now that your loved one is out of town for a specified amount of time. Use the time to focus on things you love to do but do not always get the chance to do because of your relationship, or just not having enough time on your hands.