What if you keep a journal of differnet things that you experience and the way your little fellow is changing while your husband is away. You could write a paragraph every week if you don't want to write something every day. Take what you write and add pictures of some of the things that happened during the time that you wrote about. Make sure to snap pictures every week of you and your son as well as other family and friends to add to your pages. Your journaling could even be in letter form if that would make it easier for you to think what you want to record.
Another idea that I have is to print off some calendar pages from your computer and write down anything that you want to remember on the day that it happens, or if there is to much to fit on the calendar space write it down on another paper and attach to the side of that month's calendar and then put pictures of that month on the next page and on the page around the calendar. You might even want to do more pages if there is a month that there are a lot of events you want to record. Make sure you record the day to day things that you do too not just the special occasions.
Best of luck with this project. It will be a treasure to your husband and to yourself and your son.