K.
They have shirts that say BIG GUY and little guy, you can buy him those and put the shirt that says Big Guy on top, which he won't be so thrilled with but tell him there is a card or something in it and make him lift the shirt and have the little guy shirt showing on the bottom...
Good luck.
Also I don't know if he is a fan of Family Guy but there is a sound clip of Stewie and his mom is still pregnant with him, he is giving updates and he says something like day 83 "I've grown another finger, add that to yesterdays count that makes 11.... (We used this for a dvd my husband did of my son's first year)