Thank you for serving. you sacrifice is appreciated. First, kudos for keeping in contact. Trying to remain a parent in the AOR is challenging to say the least. I guess the best way to handle your homecoming is based on how your son is handling your deployment. Is he still talking about you, or does he try to get off the phone when you get on? At 5 he is aware of what is going on, but is he as understanding as a 5 year old can be?
Without knowing you situation intimately I can only provide a few vague ideas, but these may get you thinking about something that is practical for your situation. First and foremost, please do not expect your son to respond to you as an authority figure, but more like a child would respond to the aunt or uncle who they are physically visiting for the first time in a year or more. The relationship is a level 2 communication stage (meaning it is more than "how's the weather, but less than his true inner thoughts.) A level 2 communication is a conversation in which both people try to please the other rather than be honest about emotions and personal thoughts. The best way to remedy this situation is "date nights". Take him out and talk about his fears when you gone, his fears with Daddy still being gone, give him insight to what you were doing (obviously, use wisdom on what you share), find out how it was to live with his aunt...you get the drift.
Second, I am not sure if this is possible, but try to spend a few nights at the place he has been living while you were gone. Put yourself in his environment so he knows you want to meet him where he is. This will give you an opportunity to get some sleep while knowing someone is still with your son and it will allow you to see his interactions with his aunt. I think the scariest thing about a homecoming is dealing with the "How much have they changed?", "Do they still remember me?" questions. Taking a few days to answer these questions before moving him from what is comfortable may be a great start to your homecoming.
Third, let him pick something he would like to do when you get home and make it happen. Ask him next time you talk to him. This will show him in a way that a 5 y/o understands that you love him and are willing to do something for him. Let him be king for the day and try not to control the day. Take a digital camera and take lots of pics in fun places while you are out. The pics (especially of you together) will be a reminder to him of how special you think he is.
Finally, have his aunt start talking to him about his feelings towards you and the changes that will be happening when you come home. Hopefully, his aunt is the type who will hear the bad but encourage good. Have her help him create a paper chain, mark up a calendar, or other mode of tangible days until you come home, then talk it up. Prepare him. My daughter loves to get decked out in her Daddy shirts and carry an American Flag when my husband comes home, maybe your son will enjoy something similar. Let him have pride in you and what you do! Good luck with your homecoming!