S.W.
If you filled a jar or other container with them, maybe you could put his birthday present at the bottom - like a gift card to his favorite store or restaurant. Then, once you have the word list finalize, divide the number of words by the number of days until his birthday so that on the last day, he pulls out the last words and finds his gift at the bottom. If you don't want to give him a gift card or something that will fit in the container, then write a hint or the location of a hidden gift in the container and make him go find it.