Hey, you know what would be a cool progress pic? You and your son with locked hands across a table, like you're about to arm wrestle, each flexing his biceps. You wouldn't have to show faces if you didn't want to. Take a pic like that every six months in the same position, wearing the same T-shirts. Would be a neat photo montage of growing arms and it would give you both something to look forward to.
For instance, I have a hideous photo of me taken at the base of Mt. St. Helens a little over a year ago. I'm going to go back to the very same place and have the same photo taken when I've reached my goal. The only trick will be finding identical clothes many sizes smaller.