He was finishing up his dinner, and then he raised his hands up in the air! I made the sign back and said "all done?" And then he proceeded to windshield-wiper his high chair tray, which was previously known Andrew-ese for "all done."
Incredible! I am so excited!
(See the sign for "all done" here: http://www.babysignlanguage.com/dictionary/a/all-done/)