Take a couple pictures of him each month on the same date, like the 1st of each month or something. It will really show you how much he changes over a small period of time and it's pretty cool when you have a history of him. I was doing this with my fish until we decided to buy a house and everything got super messy and busy with life for awhile. It's just now settling down but I have missed about 9 months of taking photos and I'm kicking myself. Even looking at my green terror avatar - he's probably put 2 inches on since that and filled out. It will really help you see the head growth too.
Ok - I looked back, not 9 months of missed photos, but it sure has felt like it...